diff --git a/Sprint-1/fix/median.js b/Sprint-1/fix/median.js
index 5c5b796e1..bdf5b07f3 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 sortedList = [...list].sort((a, b) => a - b);
+  const middleIndex = Math.floor(sortedList.length / 2);
+
+  if (sortedList.length % 2 === 1) {
+    return sortedList[middleIndex];
+  }
+
+  return (sortedList[middleIndex - 1] + sortedList[middleIndex]) / 2;
 }
 
 module.exports = calculateMedian;
diff --git a/Sprint-1/implement/dedupe.js b/Sprint-1/implement/dedupe.js
index 781e8718a..935d8a708 100644
--- a/Sprint-1/implement/dedupe.js
+++ b/Sprint-1/implement/dedupe.js
@@ -1 +1,5 @@
-function dedupe() {}
+function dedupe(values) {
+  return [...new Set(values)];
+}
+
+module.exports = dedupe;
diff --git a/Sprint-1/implement/dedupe.test.js b/Sprint-1/implement/dedupe.test.js
index d7c8e3d8e..83157f903 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");
+it("returns an empty array when given 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
+it("returns a copy of an array with no duplicates", () => {
+  const values = [1, "a", 2, "b"];
+  const result = dedupe(values);
+
+  expect(result).toEqual(values);
+  expect(result).not.toBe(values);
+});
 
 // 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.
+it("removes duplicate numbers while preserving their first occurrences", () => {
+  const values = [5, 1, 1, 2, 3, 2, 5, 8];
+  const result = dedupe(values);
+
+  expect(result).toEqual([5, 1, 2, 3, 8]);
+  expect(result).not.toBe(values);
+});
+
+it("removes duplicate strings while preserving their first occurrences", () => {
+  const values = ["a", "b", "a", "c", "b"];
+  const result = dedupe(values);
+
+  expect(result).toEqual(["a", "b", "c"]);
+  expect(result).not.toBe(values);
+});
diff --git a/Sprint-1/implement/describe-median.js b/Sprint-1/implement/describe-median.js
index 3381cfc0a..8b5c9d237 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..ec087ae2b 100644
--- a/Sprint-1/implement/describe-median.test.js
+++ b/Sprint-1/implement/describe-median.test.js
@@ -20,13 +20,32 @@ 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"');
+it("returns the median for [1, 2, 3]", () => {
+  expect(describeMedian([1, 2, 3])).toEqual("The median is 2");
+});
 
 // Given an empty array
 // When passed to describeMedian
 // Then it should return "Could not calculate a median: calculateMedian requires a non-empty array"
+it("returns an error description when given an empty array", () => {
+  try {
+    expect(describeMedian([])).toBe(
+      "Could not calculate a median: calculateMedian requires a non-empty array"
+    );
+  } catch (error) {
+    throw new Error(`Empty-array case failed: ${error.message}`);
+  }
+});
 
 // 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"
+it("returns an error description for an array of strings", () => {
+  try {
+    expect(describeMedian(["banana", "apple", "orange"])).toBe(
+      "Could not calculate a median: calculateMedian requires an array of numbers"
+    );
+  } catch (error) {
+    throw new Error(`String-array case failed: ${error.message}`);
+  }
+});
diff --git a/Sprint-1/implement/max.js b/Sprint-1/implement/max.js
index 6dd76378e..c223d65bf 100644
--- a/Sprint-1/implement/max.js
+++ b/Sprint-1/implement/max.js
@@ -1,4 +1,14 @@
 function findMax(elements) {
+  if (
+    !Array.isArray(elements) ||
+    elements.some((element) => typeof element !== "number")
+  ) {
+    throw new Error("findMax requires an array of numbers");
+  }
+  return elements.reduce(
+    (max, element) => (element > max ? element : max),
+    -Infinity
+  );
 }
 
 module.exports = findMax;
diff --git a/Sprint-1/implement/max.test.js b/Sprint-1/implement/max.test.js
index ff3fbb5e5..d35f25cff 100644
--- a/Sprint-1/implement/max.test.js
+++ b/Sprint-1/implement/max.test.js
@@ -15,29 +15,71 @@ 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.todo("given an empty array, returns -Infinity");
+it("returns -Infinity for an empty array", () => {
+  expect(findMax([])).toBe(-Infinity);
+});
 
 // Given an array with one number
 // When passed to the max function
 // Then it should return that number
+it("returns 10 for a single positive number", () => {
+  expect(findMax([10])).toBe(10);
+});
+
+it("returns the largest value in a positive-number array", () => {
+  expect(findMax([10, 20, 5])).toBe(20);
+});
 
 // Given an array with both positive and negative numbers
 // When passed to the max function
 // Then it should return the largest number overall
+it("returns 200 for an array of positive and negative numbers", () => {
+  expect(findMax([10, -10, 200, -15])).toEqual(200);
+});
+
+it("returns Infinity when Infinity is the largest value", () => {
+  expect(findMax([10, Infinity, -15])).toBe(Infinity);
+});
 
 // Given an array with just negative numbers
 // When passed to the max function
 // Then it should return the closest one to zero
+it("returns -1 for an array of negative numbers", () => {
+  expect(findMax([-1, -10, -200, -15, -30])).toEqual(-1);
+});
+
+it("returns the number for a single negative value", () => {
+  expect(findMax([-10])).toBe(-10);
+});
 
 // Given an array with decimal numbers
 // When passed to the max function
 // Then it should return the largest decimal number
+it("returns 200.99 for an array of decimal numbers", () => {
+  expect(findMax([1.5, 2.5, 200.99, 15.01, 30.1])).toBe(200.99);
+});
 
 // 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")
+it("throws for an array of strings", () => {
+  expect(() => findMax(["Hello", "There", "Weekend"])).toThrow(
+    "findMax requires an array of numbers"
+  );
+});
+
+it("throws for an array mixing strings and numbers", () => {
+  expect(() => findMax(["Hello", 10, 20])).toThrow(
+    "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")
+it("throws when the input is not an array", () => {
+  expect(() => findMax("Hello")).toThrow(
+    "findMax requires an array of numbers"
+  );
+});
diff --git a/Sprint-1/implement/mean.js b/Sprint-1/implement/mean.js
index 36909ea1f..fc69f13c6 100644
--- a/Sprint-1/implement/mean.js
+++ b/Sprint-1/implement/mean.js
@@ -1,3 +1,21 @@
-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") {
+      throw new Error("calculateMean requires an array of numbers");
+    }
+  }
+
+  const sum = list.reduce((a, b) => a + b, 0);
+
+  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..247a26ec0 100644
--- a/Sprint-1/implement/mean.test.js
+++ b/Sprint-1/implement/mean.test.js
@@ -22,29 +22,56 @@ 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");
+it("returns the mean in a positive-number array", () => {
+  expect(calculateMean([1, 2, 6])).toBe(3);
+});
 
 // Given an array with a single number
 // When passed to calculateMean
 // Then it should return that number
+it("returns 10 for a single positive number", () => {
+  expect(calculateMean([10])).toBe(10);
+});
 
 // Given an array containing negative or decimal numbers
 // When passed to calculateMean
 // Then it should return the correct mean
+it("returns -3 for an array of negative numbers", () => {
+  expect(calculateMean([-1, -2, -6])).toEqual(-3);
+});
 
 // Given an empty array
 // When passed to calculateMean
 // Then it should throw Error("calculateMean requires a non-empty array")
+it("throws for an empty array", () => {
+  expect(() => calculateMean([])).toThrow(
+    "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")
+it("throws when the input is not an array", () => {
+  expect(() => calculateMean("Hello")).toThrow(
+    "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")
+it("throws when the input has no argument", () => {
+  expect(() => calculateMean("")).toThrow(
+    "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")
+it("throws for an array containing a non-number value", () => {
+  expect(() => calculateMean(["2", 10, 20])).toThrow(
+    "calculateMean requires an array of numbers"
+  );
+});
diff --git a/Sprint-1/implement/sum.js b/Sprint-1/implement/sum.js
index 9062aafe3..839065de3 100644
--- a/Sprint-1/implement/sum.js
+++ b/Sprint-1/implement/sum.js
@@ -1,4 +1,13 @@
 function sum(elements) {
+  if (!Array.isArray(elements)) {
+    throw new Error("sum requires an array of numbers");
+  }
+
+  if (elements.some((element) => typeof element !== "number")) {
+    throw new Error("sum requires an array of numbers");
+  }
+
+  return elements.reduce((total, element) => total + element, 0);
 }
 
 module.exports = sum;
diff --git a/Sprint-1/implement/sum.test.js b/Sprint-1/implement/sum.test.js
index a288e0f0a..dc940172e 100644
--- a/Sprint-1/implement/sum.test.js
+++ b/Sprint-1/implement/sum.test.js
@@ -13,24 +13,41 @@ 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")
+it("returns 0 for an empty array", () => {
+  expect(sum([])).toBe(0);
+});
 
 // Given an array with just one number
 // When passed to the sum function
 // Then it should return that number
+it("returns 10 for a single positive number", () => {
+  expect(sum([10])).toBe(10);
+});
 
 // Given an array containing negative numbers
 // When passed to the sum function
 // Then it should still return the correct total sum
+it("returns -9 for an array of negative numbers", () => {
+  expect(sum([-1, -2, -6])).toEqual(-9);
+});
 
 // Given an array with decimal/float numbers
 // When passed to the sum function
 // Then it should return the correct total sum
+it("returns 220 for an array of decimal numbers", () => {
+  expect(sum([1.5, 2.5, 200.99, 15.01])).toBe(220);
+});
 
 // 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")
+it("throws when the input is not an array", () => {
+  expect(() => sum("Hello")).toThrow("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")
+it("throws when the input is not an array", () => {
+  expect(() => sum("Hey", 42, "")).toThrow("sum requires an array of numbers");
+});
diff --git a/Sprint-1/refactor/includes.js b/Sprint-1/refactor/includes.js
index 29dad81f0..8c9ae2e66 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;
     }