diff --git a/Sprint-1/fix/median.js b/Sprint-1/fix/median.js
index 5c5b796e1..53b578809 100644
--- a/Sprint-1/fix/median.js
+++ b/Sprint-1/fix/median.js
@@ -6,9 +6,28 @@
 // or contains values that aren't numbers (the function is expected to throw - see the tests).
 
 function calculateMedian(list) {
-  const middleIndex = Math.floor(list.length / 2);
-  const median = list.splice(middleIndex, 1)[0];
-  return median;
+  if (!Array.isArray(list)) {
+    throw new Error("calculateMedian requires an array of numbers");
+  }
+
+  if (list.length === 0) {
+    throw new Error("calculateMedian requires a non-empty array");
+  }
+
+  for (const item of list) {
+    if (typeof item !== "number") {
+      throw new Error("calculateMedian requires an array of numbers");
+    }
+  }
+
+  const sorted = list.slice().sort((a, b) => a - b);
+  const middleIndex = Math.floor(sorted.length / 2);
+
+  if (sorted.length % 2 === 0) {
+    return (sorted[middleIndex - 1] + sorted[middleIndex]) / 2;
+  }
+
+  return sorted[middleIndex];
 }
 
 module.exports = calculateMedian;
diff --git a/Sprint-1/implement/dedupe.js b/Sprint-1/implement/dedupe.js
index 781e8718a..47c5ef797 100644
--- a/Sprint-1/implement/dedupe.js
+++ b/Sprint-1/implement/dedupe.js
@@ -1 +1,5 @@
-function dedupe() {}
+function dedupe(arr) {
+  return arr.filter((item, index) => arr.indexOf(item) === index);
+}
+
+module.exports = dedupe;
diff --git a/Sprint-1/implement/dedupe.test.js b/Sprint-1/implement/dedupe.test.js
index d7c8e3d8e..eddd4e9d4 100644
--- a/Sprint-1/implement/dedupe.test.js
+++ b/Sprint-1/implement/dedupe.test.js
@@ -16,13 +16,37 @@ E.g. dedupe([1, 2, 1]) returns [1, 2]
 // Given an empty array
 // When passed to the dedupe function
 // Then it should return an empty array
-test.todo("given an empty array, it returns an empty array");
+test("Given an empty array, should return an empty array", () => {
+  expect(dedupe([])).toEqual([]);
+});
 
 // Given an array with no duplicates
 // When passed to the dedupe function
 // Then it should return a copy of the original array
+test("Given an array with no duplicates, should return a copy of the original array", () => {
+  const original = [5, 1, 2, 3, 8];
+  const result = dedupe(original);
+
+  expect(result).toEqual([5, 1, 2, 3, 8]);
+  expect(result).not.toBe(original);
+});
 
 // Given an array of strings or numbers
 // When passed to the dedupe function
-// Then it should return a new array with duplicates removed while preserving the 
+// Then it should return a new array with duplicates removed while preserving the
 // first occurrence of each element from the original array.
+test("Given an array of strings, removes duplicates and keeps first occurrence", () => {
+  expect(dedupe(["car", "plane", "car", "train", "plane"])).toEqual([
+    "car",
+    "plane",
+    "train",
+  ]);
+});
+
+test("Given an array of numbers, removes duplicates and keeps first occurrence", () => {
+  expect(dedupe([5, 1, 1, 2, 3, 2, 5, 8])).toEqual([5, 1, 2, 3, 8]);
+});
+
+test("Given [1, 2, 1], returns [1, 2]", () => {
+  expect(dedupe([1, 2, 1])).toEqual([1, 2]);
+});
diff --git a/Sprint-1/implement/describe-median.js b/Sprint-1/implement/describe-median.js
index 3381cfc0a..f6fdd5660 100644
--- a/Sprint-1/implement/describe-median.js
+++ b/Sprint-1/implement/describe-median.js
@@ -20,6 +20,13 @@ function calculateMedian(list) {
 }
 
 // Implement this function. See describe-median.test.js for the acceptance criteria.
-function describeMedian(list) {}
+function describeMedian(list) {
+  try {
+    const median = calculateMedian(list);
+    return `The median is ${median}`;
+  } catch (error) {
+    return `Could not calculate a median: ${error.message}`;
+  }
+}
 
 module.exports = describeMedian;
diff --git a/Sprint-1/implement/describe-median.test.js b/Sprint-1/implement/describe-median.test.js
index e7d18a264..f04bc5929 100644
--- a/Sprint-1/implement/describe-median.test.js
+++ b/Sprint-1/implement/describe-median.test.js
@@ -20,13 +20,31 @@ const describeMedian = require("./describe-median.js");
 // Given an array of numbers
 // When passed to describeMedian
 // Then it should return "The median is " followed by the median
-// Delete this test.todo and replace it with a test.
-test.todo('given [1, 2, 3], returns "The median is 2"');
+test('Given an array of numbers, returns "The median is " followed by the median', () => {
+  expect(describeMedian([5, 6, 7])).toEqual("The median is 6");
+});
 
 // Given an empty array
 // When passed to describeMedian
 // Then it should return "Could not calculate a median: calculateMedian requires a non-empty array"
+test("Given an empty array, returns could-not-calculate message", () => {
+  expect(describeMedian([])).toEqual(
+    "Could not calculate a median: calculateMedian requires a non-empty array"
+  );
+});
 
 // Given something that isn't an array of numbers, e.g. "banana"
 // When passed to describeMedian
 // Then it should return "Could not calculate a median: calculateMedian requires an array of numbers"
+test("Given non-numbers, returns could-not-calculate message", () => {
+  expect(describeMedian(["banana"])).toEqual(
+    "Could not calculate a median: calculateMedian requires an array of numbers"
+  );
+});
+
+test("Given a string, returns could-not-calculate message", () => {
+  expect(describeMedian("banana")).toEqual(
+    "Could not calculate a median: calculateMedian requires an array of numbers"
+  );
+});
+
diff --git a/Sprint-1/implement/max.js b/Sprint-1/implement/max.js
index 6dd76378e..ddf6b202d 100644
--- a/Sprint-1/implement/max.js
+++ b/Sprint-1/implement/max.js
@@ -1,4 +1,26 @@
+
 function findMax(elements) {
+  if (!Array.isArray(elements)) {
+    throw new Error("findMax requires an array of numbers");
+  }
+
+  for (const item of elements) {
+    if (typeof item !== "number" || Number.isNaN(item)) { 
+      throw new Error("findMax requires an array of numbers");
+    }
+  }
+
+  if (elements.length === 0) {
+    return -Infinity;
+  }
+
+  let max = elements[0];
+  for (const item of elements) {
+    if (item > max) {
+      max = item;
+    }
+  }
+  return max;
 }
 
 module.exports = findMax;
diff --git a/Sprint-1/implement/max.test.js b/Sprint-1/implement/max.test.js
index ff3fbb5e5..0485062c7 100644
--- a/Sprint-1/implement/max.test.js
+++ b/Sprint-1/implement/max.test.js
@@ -15,29 +15,69 @@ const findMax = require("./max.js");
 // Given an empty array
 // When passed to the max function
 // Then it should return -Infinity
-// Delete this test.todo and replace it with a test.
-test.todo("given an empty array, returns -Infinity");
+test("given an empty array, returns -Infinity", () => {
+  expect(findMax([])).toEqual(-Infinity);
+});
 
 // Given an array with one number
 // When passed to the max function
 // Then it should return that number
+test("Given an array with one number, should return thant number", () => {
+  expect(findMax([10])).toEqual(10);
+});
 
 // Given an array with both positive and negative numbers
 // When passed to the max function
 // Then it should return the largest number overall
+test("Given an array with both positive and negative numbers, should return the largest number overall", () => {
+  expect(findMax([30, -7, 1, 45, -70, 10, -10])).toEqual(45);
+});
 
 // Given an array with just negative numbers
 // When passed to the max function
 // Then it should return the closest one to zero
+test("Given an array with just negative numbers, should return the closest one to zero", () => {
+  expect(findMax([-15, -10, -7, -20, -70])).toEqual(-7);
+});
 
 // Given an array with decimal numbers
 // When passed to the max function
 // Then it should return the largest decimal number
+test("Given an array with decimal numbers, should return the largest decimal number", () => {
+  expect(findMax([3.5, 2.7, 1.9, 5.1])).toEqual(5.1);
+});
 
 // Given an array containing a value that isn't a number
 // When passed to the max function
 // Then it should throw Error("findMax requires an array of numbers")
-
+test("Throws an error when array contains a value that isn't a number", () => {
+  expect(() => findMax(["plane", 10, "car", 60, 10])).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
 // Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the max function
 // Then it should throw Error("findMax requires an array of numbers")
+test("Throws an error when the input is not an array at all", () => {
+  expect(() => findMax("hey")).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
+
+test("Throws an error when given a number", () => {
+  expect(() => findMax(36)).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
+
+test("Throws an error when called with no argument", () => {
+  expect(() => findMax()).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
+
+test("Throws an error when array contains NaN", () => {
+  expect(() => findMax([1, NaN, 3])).toThrow(
+    new Error("findMax requires an array of numbers")
+  );
+});
diff --git a/Sprint-1/implement/mean.js b/Sprint-1/implement/mean.js
index 36909ea1f..a048fdf08 100644
--- a/Sprint-1/implement/mean.js
+++ b/Sprint-1/implement/mean.js
@@ -1,3 +1,24 @@
-function calculateMean(list) {}
+function calculateMean(list) {
+  if (!Array.isArray(list)) {
+    throw new Error("calculateMean requires an array of numbers");
+  }
+
+  if (list.length === 0) {
+    throw new Error("calculateMean requires a non-empty array");
+  }
+
+  for (const item of list) {
+    if (typeof item !== "number" || !Number.isFinite(item)) {
+      throw new Error("calculateMean requires an array of numbers");
+    }
+  }
+
+  let sum = 0;
+  for (const num of list) {
+    sum += num;
+  }
+  return sum / list.length;
+}
 
 module.exports = calculateMean;
+
diff --git a/Sprint-1/implement/mean.test.js b/Sprint-1/implement/mean.test.js
index 847b88676..388c0fa73 100644
--- a/Sprint-1/implement/mean.test.js
+++ b/Sprint-1/implement/mean.test.js
@@ -22,29 +22,77 @@ const calculateMean = require("./mean.js");
 // Given an array of numbers
 // When passed to calculateMean
 // Then it should return their mean
-// Delete this test.todo and replace it with a test.
-test.todo("given [1, 2, 6], returns 3");
+
+test("Given an array of numbers, should return their mean", () => {
+  expect(calculateMean([3, 7, 50,40])).toEqual(25);
+});
 
 // Given an array with a single number
 // When passed to calculateMean
 // Then it should return that number
+test("Given an array with a single number, should return that number", () => {
+  expect(calculateMean([10])).toEqual(10);
+});
 
 // Given an array containing negative or decimal numbers
 // When passed to calculateMean
 // Then it should return the correct mean
+test("Given an array containing negative numbers, should return the correct mean", () => {
+  expect(calculateMean([-3, -50, -7, -40])).toEqual(-25);
+});
+
+test("Given an array containing decimal numbers, should return the correct mean", () => {
+  expect(calculateMean([2.2, 1.9, 3.5, 4.1])).toBeCloseTo(2.925);
+});
+
+test("Given an array containing both negative and decimal numbers, should return the correct mean", () => {
+  expect(calculateMean([2.2, -1.9, 3.5, -4.1])).toBeCloseTo(-0.075);
+});
 
 // Given an empty array
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires a non-empty array")
+test("Throws an error when given an empty array", () => {
+  expect(() => calculateMean([])).toThrow(
+    new Error("calculateMean requires a non-empty array")
+  );
+});
 
 // Given a value that isn't an array, e.g. "banana", 42, null or {}
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires an array of numbers")
+test("Throws an error when given a value that isn't an array", () => {
+  expect(() => calculateMean(["banana"])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
 
 // Given no argument at all
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires an array of numbers")
+test("Throws an error when given no argument at all", () => {
+  expect(() => calculateMean()).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
 
 // Given an array containing a non-number value, e.g. [1, "2", 3]
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires an array of numbers")
+test("Throws an error when given an array containing a non-number value", () => {
+  expect(() => calculateMean([1, "2", 3])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
+
+test("Throws an error when array contains Infinity or -Infinity", () => {
+  expect(() => calculateMean([Infinity, 0, -Infinity])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
+
+test("Throws an error when array contains NaN", () => {
+  expect(() => calculateMean([0, NaN])).toThrow(
+    new Error("calculateMean requires an array of numbers")
+  );
+});
diff --git a/Sprint-1/implement/sum.js b/Sprint-1/implement/sum.js
index 9062aafe3..a1474f1f0 100644
--- a/Sprint-1/implement/sum.js
+++ b/Sprint-1/implement/sum.js
@@ -1,4 +1,19 @@
 function sum(elements) {
+  if (!Array.isArray(elements)) {
+    throw new Error("sum requires an array of numbers");
+  }
+
+  for (const item of elements) {
+    if (typeof item !== "number") {
+      throw new Error("sum requires an array of numbers");
+    }
+  }
+
+  let total = 0;
+  for (const item of elements) {
+    total += item;
+  }
+  return total;
 }
 
 module.exports = sum;
diff --git a/Sprint-1/implement/sum.test.js b/Sprint-1/implement/sum.test.js
index a288e0f0a..ef591a15f 100644
--- a/Sprint-1/implement/sum.test.js
+++ b/Sprint-1/implement/sum.test.js
@@ -13,24 +13,46 @@ const sum = require("./sum.js");
 // Given an empty array
 // When passed to the sum function
 // Then it should return 0
-test.todo("given an empty array, returns 0")
+test("Given an empty array, should return 0", () => {
+  expect(sum([])).toEqual(0);
+});
 
 // Given an array with just one number
 // When passed to the sum function
 // Then it should return that number
+test("Given an array with just one number, should return that number", () => {
+  expect(sum([10])).toEqual(10);
+});
 
 // Given an array containing negative numbers
 // When passed to the sum function
 // Then it should still return the correct total sum
+test("Given an array containing negative numbers, should still return the correct total sum", () => {
+  expect(sum([-3, -7, -2])).toEqual(-12);
+});
 
 // Given an array with decimal/float numbers
 // When passed to the sum function
 // Then it should return the correct total sum
+test("Given an array with decimal/float numbers, should return the correct total sum", () => {
+  expect(sum([1.3, 2.7, 3.5])).toEqual(7.5);
+});
 
 // Given an array containing a value that isn't a number
 // When passed to the sum function
 // Then it should throw Error("sum requires an array of numbers")
+test("Throws an error when an array containing a value that isn't a number", () => {
+  expect(() => sum([15, "car", 11])).toThrow(
+    new Error("sum requires an array of numbers")
+  );
+});
 
 // Given something that isn't an array at all, such as "hey", 42 or no argument
 // When passed to the sum function
 // Then it should throw Error("sum requires an array of numbers")
+test("Throws an error when given something that isn't an array at all", () => {
+  expect(() => sum("string")).toThrow(
+    new Error("sum requires an array of numbers")
+  );
+});
+
diff --git a/Sprint-1/refactor/includes.js b/Sprint-1/refactor/includes.js
index 29dad81f0..07a0e6877 100644
--- a/Sprint-1/refactor/includes.js
+++ b/Sprint-1/refactor/includes.js
@@ -1,8 +1,7 @@
 // Refactor the implementation of includes to use a for...of loop
 
 function includes(list, target) {
-  for (let index = 0; index < list.length; index++) {
-    const element = list[index];
+  for (const element of list) {
     if (element === target) {
       return true;
     }
@@ -11,3 +10,4 @@ function includes(list, target) {
 }
 
 module.exports = includes;
+
diff --git a/Sprint-1/stretch/aoc-2018-day1/solution.js b/Sprint-1/stretch/aoc-2018-day1/solution.js
index e69de29bb..e45f52fcd 100644
--- a/Sprint-1/stretch/aoc-2018-day1/solution.js
+++ b/Sprint-1/stretch/aoc-2018-day1/solution.js
@@ -0,0 +1,44 @@
+
+import fs from "node:fs";
+
+//  Puzzle input
+// trim() removes leading/trailing whitespace so split("\n")
+// does not create empty lines that become 0 after Number()
+const changes = fs
+  .readFileSync(new URL("./input.txt", import.meta.url), "utf8")
+  .trim()
+  .split("\n")
+  .map(Number);
+
+// Part One
+function calculateFrequency(changes) {
+  let frequency = 0;
+
+  for (const change of changes) {
+    frequency += change;
+  }
+
+  return frequency;
+}
+
+// Part Two
+function findRepeatedFrequency(changes) {
+  let frequency = 0;
+  const visited = new Set([0]);
+
+  while (true) {
+    for (let change of changes) {
+      frequency += change;
+
+      if (visited.has(frequency)) {
+        return frequency;
+      }
+
+      visited.add(frequency);
+    }
+  }
+}
+
+// Display answers
+console.log("Part One:", calculateFrequency(changes));
+console.log("Part Two:", findRepeatedFrequency(changes));
diff --git a/Sprint-2/debug/address.js b/Sprint-2/debug/address.js
index 940a6af83..4694634ff 100644
--- a/Sprint-2/debug/address.js
+++ b/Sprint-2/debug/address.js
@@ -12,4 +12,9 @@ const address = {
   postcode: "XYZ 123",
 };
 
-console.log(`My house number is ${address[0]}`);
+console.log(`My house number is ${address.houseNumber}`);
+
+
+/* BUG: address[0] is undefined because objects aren't indexed by number.
+ FIX: useed the property name: address.houseNumber
+console.log(`My house number is ${address.houseNumber}`);
diff --git a/Sprint-2/debug/author.js b/Sprint-2/debug/author.js
index 8c2125977..8bc5d91f0 100644
--- a/Sprint-2/debug/author.js
+++ b/Sprint-2/debug/author.js
@@ -11,6 +11,14 @@ const author = {
   alive: true,
 };
 
-for (const value of author) {
+for (const value of Object.values(author)) {
   console.log(value);
 }
+
+
+/* BUG: Objects are not iterable, so `for...of author` throws TypeError.
+ FIX: Used Object.values(author) to loop over the property values.
+for (const value of Object.values(author)) {
+  console.log(value);
+}
+*/
diff --git a/Sprint-2/debug/recipe.js b/Sprint-2/debug/recipe.js
index 6cbdd22cd..8c934063e 100644
--- a/Sprint-2/debug/recipe.js
+++ b/Sprint-2/debug/recipe.js
@@ -12,4 +12,10 @@ const recipe = {
 
 console.log(`${recipe.title} serves ${recipe.serves}
   ingredients:
-${recipe}`);
+${recipe.ingredients.join("\n")}`);
+
+
+
+// BUG: ${recipe} logs out the whole object as "[object Object]".
+// FIX: print title/serves from their properties, and ingredients with join("\n")
+// so each item is on its own line.
diff --git a/Sprint-2/implement/contains.js b/Sprint-2/implement/contains.js
index cd779308a..fa093df68 100644
--- a/Sprint-2/implement/contains.js
+++ b/Sprint-2/implement/contains.js
@@ -1,3 +1,9 @@
-function contains() {}
+function contains(obj, prop) {
+  if (obj === null || typeof obj !== "object" || Array.isArray(obj)) {
+    throw new Error("contains requires an object");
+  }
+  return Object.hasOwn(obj, prop);
+}
 
 module.exports = contains;
+
diff --git a/Sprint-2/implement/contains.test.js b/Sprint-2/implement/contains.test.js
index 719272787..bdcf0f9be 100644
--- a/Sprint-2/implement/contains.test.js
+++ b/Sprint-2/implement/contains.test.js
@@ -23,18 +23,50 @@ as an array isn't an object
 // Given an empty object
 // When passed to contains
 // Then it should return false
-test.todo("contains on empty object returns false");
+test("contains on empty object returns false", () => {
+  expect(contains({}, "a")).toBe(false);
+});
 
 // Given an object with properties
 // When passed to contains with an existing property name
 // Then it should return true
+test("returns true when property exists", () => {
+  expect(contains({ a: 1, b: 2 }, "a")).toBe(true);
+});
 
 // Given an object with properties
 // When passed to contains with a non-existent property name
 // Then it should return false
+test("returns false when property does not exist", () => {
+  expect(contains({ a: 1, b: 2 }, "c")).toBe(false);
+});
 
 // Given a value that isn't an object - an array, a string, a number,
 // null, or no argument at all
 // When passed to contains
 // Then it should throw Error("contains requires an object")
 // (careful: typeof [] and typeof null are both "object")
+
+test("throws an error when given an array", () => {
+  expect(() => contains([3, 4, 5], "a")).toThrow(
+    new Error("contains requires an object")
+  );
+});
+
+test("throws an error when given a null", () => {
+  expect(() => contains(null, "a")).toThrow(
+    new Error("contains requires an object")
+  );
+});
+
+test("throws an error when given a string", () => {
+  expect(() => contains("hellow", "a")).toThrow(
+    new Error("contains requires an object")
+  );
+});
+
+test("throws an error when given no argument", () => {
+  expect(() => contains()).toThrow(
+    new Error("contains requires an object")
+  );
+});
diff --git a/Sprint-2/implement/lookup.js b/Sprint-2/implement/lookup.js
index a6746e07f..6d82731be 100644
--- a/Sprint-2/implement/lookup.js
+++ b/Sprint-2/implement/lookup.js
@@ -1,5 +1,10 @@
-function createLookup() {
-  // implementation here
+
+function createLookup(pairs) {
+  const lookup = {};
+  for (const [country, currency] of pairs) {
+    lookup[country] = currency;
+  }
+  return lookup;
 }
 
 module.exports = createLookup;
diff --git a/Sprint-2/implement/lookup.test.js b/Sprint-2/implement/lookup.test.js
index 547e06c5a..9e3b4a4f9 100644
--- a/Sprint-2/implement/lookup.test.js
+++ b/Sprint-2/implement/lookup.test.js
@@ -1,6 +1,13 @@
 const createLookup = require("./lookup.js");
 
-test.todo("creates a country currency code lookup for multiple codes");
+test("creates a country currency code lookup for multiple codes", () => {
+  expect(
+    createLookup([
+      ["US", "USD"],
+      ["CA", "CAD"],
+    ])
+  ).toEqual({ US: "USD", CA: "CAD" });
+});
 
 /*
 
diff --git a/Sprint-2/implement/querystring.js b/Sprint-2/implement/querystring.js
index 45ec4e5f3..b94400b90 100644
--- a/Sprint-2/implement/querystring.js
+++ b/Sprint-2/implement/querystring.js
@@ -1,16 +1,46 @@
 function parseQueryString(queryString) {
   const queryParams = {};
+
   if (queryString.length === 0) {
     return queryParams;
   }
+
   const keyValuePairs = queryString.split("&");
 
   for (const pair of keyValuePairs) {
-    const [key, value] = pair.split("=");
-    queryParams[key] = value;
+    if (pair === "") {
+      continue;
+    }
+
+    const eqIndex = pair.indexOf("=");
+    let key;
+    let value;
+
+    if (eqIndex === -1) {
+      key = pair;
+      value = "";
+    } else {
+      key = pair.slice(0, eqIndex);
+      value = pair.slice(eqIndex + 1);
+    }
+
+    key = decodeURIComponent(key.replaceAll("+", " "));
+    value = decodeURIComponent(value.replaceAll("+", " "));
+
+    if (Object.hasOwn(queryParams, key)) {
+      if (Array.isArray(queryParams[key])) {
+        queryParams[key].push(value);
+      } else {
+        queryParams[key] = [queryParams[key], value];
+      }
+    } else {
+      queryParams[key] = value;
+    }
   }
 
   return queryParams;
 }
 
 module.exports = parseQueryString;
+
+
diff --git a/Sprint-2/implement/tally.js b/Sprint-2/implement/tally.js
index f47321812..09c6f37b2 100644
--- a/Sprint-2/implement/tally.js
+++ b/Sprint-2/implement/tally.js
@@ -1,3 +1,12 @@
-function tally() {}
+function tally(items) {
+  if (!Array.isArray(items)) {
+    throw new Error("tally requires an array");
+  }
+  const counts = {};
+  for (const item of items) {
+    counts[item] = (counts[item] ?? 0) + 1;
+  }
+  return counts;
+}
 
 module.exports = tally;
diff --git a/Sprint-2/implement/tally.test.js b/Sprint-2/implement/tally.test.js
index ca763c296..5f7f1bac3 100644
--- a/Sprint-2/implement/tally.test.js
+++ b/Sprint-2/implement/tally.test.js
@@ -23,12 +23,29 @@ const tally = require("./tally.js");
 // Given an empty array
 // When passed to tally
 // Then it should return an empty object
-test.todo("tally on an empty array returns an empty object");
+test("tally on an empty array returns an empty object", () => {
+  expect(tally([])).toEqual({});
+});
 
 // Given an array with duplicate items
 // When passed to tally
 // Then it should return counts for each unique item
+test("tally on an array with duplicate items, return counts for each unique item", () => {
+  expect(tally(["a", "a", "c", "b", "c", "b", "c", "e"])).toEqual({ a: 2, c: 3, b: 2, e: 1 });
+});
 
 // Given an invalid input like a string, a number, or no argument at all
 // When passed to tally
 // Then it should throw Error("tally requires an array")
+test("tally on a string, thows an error", () => {
+  expect(() => tally("hello")).toThrow(new Error("tally requires an array"));
+});
+
+test("tally on a number, thows an error", () => {
+  expect(() => tally(10)).toThrow(new Error("tally requires an array"));
+});
+
+test("tally on no argument, thows an error", () => {
+  expect(() => tally()).toThrow(new Error("tally requires an array"));
+});
+
diff --git a/Sprint-2/interpret/invert.js b/Sprint-2/interpret/invert.js
index bb353fb1f..0695413a2 100644
--- a/Sprint-2/interpret/invert.js
+++ b/Sprint-2/interpret/invert.js
@@ -6,24 +6,46 @@
 
 // E.g. invert({x : 10, y : 20}), target output: {"10": "x", "20": "y"}
 
-function invert(obj) {
+export function invert(obj) {
   const invertedObj = {};
 
   for (const [key, value] of Object.entries(obj)) {
-    invertedObj.key = value;
+    invertedObj[value] = key;
   }
 
   return invertedObj;
 }
 
-// a) What is the current return value when invert is called with { a : 1 }
+/*
+a) Current return value for invert({ a: 1 }) with the OLD bug:
+   { key: 1 }
+
+b) Current return value for invert({ a: 1, b: 2 }) with the OLD bug:
+   { key: 2 }
+   (each loop overwrites the same property named "key"; last value wins)
+
+c) Target return value for invert({ a: 1, b: 2 }):
+   { "1": "a", "2": "b" }
+
+Object.entries returns an array of [key, value] pairs for the object.
+It is needed so the loop can read both the key and the value of each property.
+
+d) invertedObj.key always writes to a property literally named "key".
+   It ignores the loop variable `key`, so keys and values are never swapped,
+   and earlier entries are overwritten. The result is a new object with only
+   that one "key" property — not the original object, and not an inverted map.
+
+e) Fixed by using invertedObj[value] = key (bracket notation with the value
+   as the new key). Tests in invert.test.js prove the fix.
+*/
+
+
+
+
+
+
 
-// b) What is the current return value when invert is called with { a: 1, b: 2 }
 
-// c) What is the target return value when invert is called with {a : 1, b: 2}
 
-// c) What does Object.entries return? Why is it needed in this program?
 
-// d) Explain why the current return value is different from the target output
 
-// e) Fix the implementation of invert (and write tests to prove it's fixed!)
diff --git a/Sprint-2/stretch/count-words.js b/Sprint-2/stretch/count-words.js
index 8e85d19d7..22f6a1292 100644
--- a/Sprint-2/stretch/count-words.js
+++ b/Sprint-2/stretch/count-words.js
@@ -26,3 +26,24 @@
 
 3. Order the results to find out which word is the most common in the input
 */
+
+function countWords(str) {
+  const counts = {};
+  if (str.trim() === "") {
+    return counts;
+  }
+
+  const words = str
+    .toLowerCase() 
+    .replaceAll(/[.,!?]/g, "") 
+    .split(/\s+/)
+    .filter((w) => w.length > 0);
+
+  for (const word of words) {
+    counts[word] = (counts[word] ?? 0) + 1;
+  }
+
+  return counts;
+}
+
+module.exports = countWords;
diff --git a/Sprint-2/stretch/mode.js b/Sprint-2/stretch/mode.js
index 9699bf307..729923ed4 100644
--- a/Sprint-2/stretch/mode.js
+++ b/Sprint-2/stretch/mode.js
@@ -12,8 +12,7 @@
 
 // The tests must still pass after your refactor. Run them again to check.
 
-function calculateMode(list) {
-  // check the input is a non-empty array of numbers
+function assertValidNumbers(list) {
   if (!Array.isArray(list)) {
     throw new Error("calculateMode requires an array of numbers");
   }
@@ -25,25 +24,32 @@ function calculateMode(list) {
       throw new Error("calculateMode requires an array of numbers");
     }
   }
+}
 
-  // track frequency of each value
-  let freqs = new Map();
-
-  for (let num of list) {
+function trackFrequencies(list) {
+  const freqs = new Map();
+  for (const num of list) {
     freqs.set(num, (freqs.get(num) || 0) + 1);
   }
+  return freqs;
+}
 
-  // Find the value with the highest frequency
+function findMode(freqs) {
   let maxFreq = 0;
   let mode;
-  for (let [num, freq] of freqs) {
+  for (const [num, freq] of freqs) {
     if (freq > maxFreq) {
       mode = num;
       maxFreq = freq;
     }
   }
-
   return mode;
 }
 
+function calculateMode(list) {
+  assertValidNumbers(list);
+  const freqs = trackFrequencies(list);
+  return findMode(freqs);
+}
+
 module.exports = calculateMode;
diff --git a/Sprint-2/stretch/till.js b/Sprint-2/stretch/till.js
index 6a08532e7..7a3daca48 100644
--- a/Sprint-2/stretch/till.js
+++ b/Sprint-2/stretch/till.js
@@ -1,5 +1,5 @@
 // totalTill takes an object representing coins in a till
-
+//
 // Given an object of coins
 // When this till object is passed to totalTill
 // Then it should return the total amount in pounds
@@ -8,24 +8,42 @@ function totalTill(till) {
   let total = 0;
 
   for (const [coin, quantity] of Object.entries(till)) {
-    total += coin * quantity;
+    const valueInPence = Number(coin.replace("p", ""));
+    total += valueInPence * quantity;
   }
 
   return `£${total / 100}`;
 }
 
+module.exports = totalTill;
+
 const till = {
   "1p": 10,
   "5p": 6,
   "50p": 4,
   "20p": 10,
 };
-const totalAmount = totalTill(till);
-
-// a) What is the target output when totalTill is called with the till object
-
-// b) Why do we need to use Object.entries inside the for...of loop in this function?
-
-// c) What does coin * quantity evaluate to inside the for...of loop?
 
-// d) Write a test for this function to check it works and then fix the implementation of totalTill
+// a) Target output when totalTill is called with the till object above:
+//    "£3.4"
+//    Calculation: (10*1) + (6*5) + (4*50) + (10*20) = 10 + 30 + 200 + 200 = 340 pence
+//    340 / 100 = 3.4 → "£3.4"
+
+// b) Object.entries(till) turns the object into an array of [key, value] pairs
+//    e.g. [["1p", 10], ["5p", 6], ...].
+//    for...of needs something iterable; Object.entries lets us read both the
+//    coin label and its quantity in each iteration.
+
+// c) Before the fix: coin * quantity was "1p" * 10 (string * number) → NaN.
+//    After the fix: valueInPence * quantity is e.g. 1 * 10, 5 * 6, 50 * 4, 20 * 10
+//    (numbers only), and those products are added into total.
+
+// d) Example test (till.test.js):
+//
+//    const totalTill = require("./till.js");
+//
+//    test("sums coins in the till as pounds", () => {
+//      expect(
+//        totalTill({ "1p": 10, "5p": 6, "50p": 4, "20p": 10 })
+//      ).toBe("£3.4");
+//    });