diff --git a/Sprint-3/1-key-errors/0.js b/Sprint-3/1-key-errors/0.js
index 653d6f5a0..103c3a3b5 100644
--- a/Sprint-3/1-key-errors/0.js
+++ b/Sprint-3/1-key-errors/0.js
@@ -1,13 +1,27 @@
 // Predict and explain first...
-//  =============> write your prediction here
+// str is a parameter in the function capitalise, so there will likely be a syntax error when trying to declare a second variable (let str) with the same name in the same scope. 
+
 
 // call the function capitalise with a string input
 // interpret the error message and figure out why an error is occurring
 
+
+//function capitalise(str) {
+ // let str = `${str[0].toUpperCase()}${str.slice(1)}`;
+  //return str;
+//}
+
+// =============> write your explanation here
+// Error message: SyntaxError: Identifier 'str' has already been declared
+// A parameter is already a variable declared inside the function's scope.
+// let cannot declare a second variable with the same name in the same scope, so
+// JavaScript rejects the whole file before running anything. That is why the
+// error appears even before capitalise is called.
+
+// =============> write your new code here
 function capitalise(str) {
-  let str = `${str[0].toUpperCase()}${str.slice(1)}`;
+  str = `${str[0].toUpperCase()}${str.slice(1)}`;
   return str;
 }
 
-// =============> write your explanation here
-// =============> write your new code here
+console.log(capitalise("hello")); // Hello
diff --git a/Sprint-3/1-key-errors/1.js b/Sprint-3/1-key-errors/1.js
index f2d56151f..aad307d76 100644
--- a/Sprint-3/1-key-errors/1.js
+++ b/Sprint-3/1-key-errors/1.js
@@ -2,19 +2,37 @@
 
 // Why will an error occur when this program runs?
 // =============> write your prediction here
+// I predict a SyntaxError, because decimalNumber is the parameter and is declared
+// again inside the function with const.
+// Even after fixing that, console.log(decimalNumber) is outside the function, where
+// decimalNumber does not exist, so it would cause a ReferenceError.
 
 // Try playing computer with the example to work out what is going on
 
+// function convertToPercentage(decimalNumber) {
+//   const decimalNumber = 0.5;
+//   const percentage = `${decimalNumber * 100}%`;
+//
+//   return percentage;
+// }
+//
+// console.log(decimalNumber);
+
+// =============> write your explanation here
+// First error: SyntaxError: Identifier 'decimalNumber' has already been declared.
+// The parameter already declares decimalNumber in the function's scope, so const
+// cannot declare it again. The hardcoded 0.5 would also ignore whatever value was passed in.
+//
+// Second error (after removing the const line): ReferenceError: decimalNumber is not defined.
+// Parameters only exist inside the function's local scope. Outside the function we
+// should call the function and log its return value instead.
+
+// Finally, correct the code to fix the problem
+// =============> write your new code here
 function convertToPercentage(decimalNumber) {
-  const decimalNumber = 0.5;
   const percentage = `${decimalNumber * 100}%`;
 
   return percentage;
 }
 
-console.log(decimalNumber);
-
-// =============> write your explanation here
-
-// Finally, correct the code to fix the problem
-// =============> write your new code here
+console.log(convertToPercentage(0.5)); // 50%
diff --git a/Sprint-3/1-key-errors/2.js b/Sprint-3/1-key-errors/2.js
index aad57f7cf..0d842c89e 100644
--- a/Sprint-3/1-key-errors/2.js
+++ b/Sprint-3/1-key-errors/2.js
@@ -1,20 +1,30 @@
-
 // Predict and explain first BEFORE you run any code...
 
 // this function should square any number but instead we're going to get an error
 
 // =============> write your prediction of the error here
+// I predict a SyntaxError, because 3 is written where a parameter name should be.
+// Parameters must be variable names, not values.
 
-function square(3) {
-    return num * num;
-}
+// function square(3) {
+//     return num * num;
+// }
 
 // =============> write the error message here
+// SyntaxError: Unexpected number
 
 // =============> explain this error message here
+// When declaring a function, the brackets hold parameter names: placeholders that
+// receive values when the function is called. JavaScript expected a name there and
+// found the number 3, which is not a valid name, so it reports an "Unexpected number".
+// The body also uses num, which is never declared, so num should be the parameter.
 
 // Finally, correct the code to fix the problem
 
 // =============> write your new code here
+function square(num) {
+  return num * num;
+}
 
+console.log(square(3)); // 9
 
diff --git a/Sprint-3/2-mandatory-debug/0.js b/Sprint-3/2-mandatory-debug/0.js
index b27511b41..a8a020a20 100644
--- a/Sprint-3/2-mandatory-debug/0.js
+++ b/Sprint-3/2-mandatory-debug/0.js
@@ -1,14 +1,30 @@
 // Predict and explain first...
 
 // =============> write your prediction here
+// I predict it prints 320 on one line, then
+// "The result of multiplying 10 and 32 is undefined".
+// multiply logs the answer but never returns it.
 
-function multiply(a, b) {
-  console.log(a * b);
-}
-
-console.log(`The result of multiplying 10 and 32 is ${multiply(10, 32)}`);
+// function multiply(a, b) {
+//   console.log(a * b);
+// }
+//
+// console.log(`The result of multiplying 10 and 32 is ${multiply(10, 32)}`);
 
 // =============> write your explanation here
+// Actual output:
+// 320
+// The result of multiplying 10 and 32 is undefined
+// console.log only prints a value to the terminal; it does not send it back to
+// the caller. A function with no return statement returns undefined, so the
+// template literal receives undefined. The 320 appears first because multiply
+// runs (and logs) while the template literal is being built.
 
 // Finally, correct the code to fix the problem
-//  =============> write your new code here
+// =============> write your new code here
+function multiply(a, b) {
+  return a * b;
+}
+
+console.log(`The result of multiplying 10 and 32 is ${multiply(10, 32)}`);
+// The result of multiplying 10 and 32 is 320
\ No newline at end of file
diff --git a/Sprint-3/2-mandatory-debug/1.js b/Sprint-3/2-mandatory-debug/1.js
index 37cedfbcf..a12a4d687 100644
--- a/Sprint-3/2-mandatory-debug/1.js
+++ b/Sprint-3/2-mandatory-debug/1.js
@@ -1,13 +1,25 @@
 // Predict and explain first...
 //  =============> write your prediction here
+// I predict "The sum of 10 and 32 is undefined", because return is on its own line
+// and a + b is on the next line.
 
-function sum(a, b) {
-  return;
-  a + b;
-}
-
-console.log(`The sum of 10 and 32 is ${sum(10, 32)}`);
+// function sum(a, b) {
+//   return;
+//   a + b;
+// }
+//
+// console.log(`The sum of 10 and 32 is ${sum(10, 32)}`);
 
 // =============> write your explanation here
+// Actual output: The sum of 10 and 32 is undefined
+// return; with nothing after it ends the function immediately and returns undefined.
+// The line a + b is never reached. return must be followed by the value on the same line.
+
 // Finally, correct the code to fix the problem
 //  =============> write your new code here
+function sum(a, b) {
+  return a + b;
+}
+
+console.log(`The sum of 10 and 32 is ${sum(10, 32)}`);
+// The sum of 10 and 32 is 42
\ No newline at end of file
diff --git a/Sprint-3/2-mandatory-debug/2.js b/Sprint-3/2-mandatory-debug/2.js
index 57d3f5dc3..1ba958e87 100644
--- a/Sprint-3/2-mandatory-debug/2.js
+++ b/Sprint-3/2-mandatory-debug/2.js
@@ -2,23 +2,43 @@
 
 // Predict the output of the following code:
 // =============> Write your prediction here
+// I predict all three lines will say the last digit is 3, because getLastDigit
+// has no parameter and always uses the global num, which is 103.
 
-const num = 103;
-
-function getLastDigit() {
-  return num.toString().slice(-1);
-}
-
-console.log(`The last digit of 42 is ${getLastDigit(42)}`);
-console.log(`The last digit of 105 is ${getLastDigit(105)}`);
-console.log(`The last digit of 806 is ${getLastDigit(806)}`);
+// const num = 103;
+//
+// function getLastDigit() {
+//   return num.toString().slice(-1);
+// }
+//
+// console.log(`The last digit of 42 is ${getLastDigit(42)}`);
+// console.log(`The last digit of 105 is ${getLastDigit(105)}`);
+// console.log(`The last digit of 806 is ${getLastDigit(806)}`);
 
 // Now run the code and compare the output to your prediction
 // =============> write the output here
+// The last digit of 42 is 3
+// The last digit of 105 is 3
+// The last digit of 806 is 3
+
 // Explain why the output is the way it is
 // =============> write your explanation here
+// getLastDigit is declared with no parameters, so the arguments 42, 105 and 806
+// are ignored. Inside the function, num refers to the global constant 103.
+// 103.toString() is "103" and .slice(-1) takes the last character, "3", every time.
+
 // Finally, correct the code to fix the problem
 // =============> write your new code here
+// Give the function a parameter so it uses the number passed in, and remove the global num.
+function getLastDigit(num) {
+  return num.toString().slice(-1);
+}
+
+console.log(`The last digit of 42 is ${getLastDigit(42)}`); // 2
+console.log(`The last digit of 105 is ${getLastDigit(105)}`); // 5
+console.log(`The last digit of 806 is ${getLastDigit(806)}`); // 6
 
 // This program should tell the user the last digit of each number.
 // Explain why getLastDigit is not working properly - correct the problem
+// It ignored its input because it had no parameter. Adding num as a parameter means
+// each call works on the number it was given.
diff --git a/Sprint-3/3-mandatory-implement/1-bmi.js b/Sprint-3/3-mandatory-implement/1-bmi.js
index 58b1085f1..9fd3521ec 100644
--- a/Sprint-3/3-mandatory-implement/1-bmi.js
+++ b/Sprint-3/3-mandatory-implement/1-bmi.js
@@ -16,4 +16,11 @@
 
 function calculateBMI(weight, height) {
   // return the BMI of someone based off their weight and height
+  const heightSquared = height * height;
+  const bmi = weight / heightSquared;
+  return bmi.toFixed(1); // toFixed returns a string rounded to 1 decimal place
 }
+
+console.log(calculateBMI(70, 1.73)); // "23.4"
+console.log(calculateBMI(85, 1.8)); // "26.2"
+console.log(calculateBMI(50, 1.6)); // "19.5"
\ No newline at end of file
diff --git a/Sprint-3/3-mandatory-implement/2-cases.js b/Sprint-3/3-mandatory-implement/2-cases.js
index 5b0ef77ad..1289f07ed 100644
--- a/Sprint-3/3-mandatory-implement/2-cases.js
+++ b/Sprint-3/3-mandatory-implement/2-cases.js
@@ -14,3 +14,12 @@
 // You will need to come up with an appropriate name for the function
 // Use the MDN string documentation to help you find a solution
 // This might help https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/toUpperCase
+
+function toUpperSnakeCase(text) {
+  // replaceAll swaps every space for an underscore, then toUpperCase capitalises every letter
+  return text.replaceAll(" ", "_").toUpperCase();
+}
+
+console.log(toUpperSnakeCase("hello there")); // "HELLO_THERE"
+console.log(toUpperSnakeCase("lord of the rings")); // "LORD_OF_THE_RINGS"
+console.log(toUpperSnakeCase("code your future")); // "CODE_YOUR_FUTURE"
\ No newline at end of file
diff --git a/Sprint-3/3-mandatory-implement/3-to-pounds.js b/Sprint-3/3-mandatory-implement/3-to-pounds.js
index 10754da73..e3ecbda86 100644
--- a/Sprint-3/3-mandatory-implement/3-to-pounds.js
+++ b/Sprint-3/3-mandatory-implement/3-to-pounds.js
@@ -4,3 +4,32 @@
 // You will need to declare a function called toPounds with an appropriately named parameter.
 
 // You should call this function a number of times to check it works for different inputs
+
+function toPounds(penceString) {
+  // Remove the trailing "p", e.g. "399p" -> "399"
+  const penceStringWithoutTrailingP = penceString.substring(
+    0,
+    penceString.length - 1,
+  );
+
+  // Pad with zeros so there is always at least one pounds digit and two pence digits
+  const paddedPenceNumberString = penceStringWithoutTrailingP.padStart(3, "0");
+
+  // Everything except the last two digits is pounds
+  const pounds = paddedPenceNumberString.substring(
+    0,
+    paddedPenceNumberString.length - 2,
+  );
+
+  // The last two digits are pence
+  const pence = paddedPenceNumberString
+    .substring(paddedPenceNumberString.length - 2)
+    .padEnd(2, "0");
+
+  return `£${pounds}.${pence}`;
+}
+
+console.log(toPounds("399p")); // £3.99
+console.log(toPounds("5p")); // £0.05
+console.log(toPounds("50p")); // £0.50
+console.log(toPounds("12345p")); // £123.45
\ No newline at end of file
diff --git a/Sprint-3/4-mandatory-interpret/time-format.js b/Sprint-3/4-mandatory-interpret/time-format.js
index c0dd9c9a5..201295510 100644
--- a/Sprint-3/4-mandatory-interpret/time-format.js
+++ b/Sprint-3/4-mandatory-interpret/time-format.js
@@ -22,17 +22,29 @@ function formatTimeDisplay(seconds) {
 
 // a) When formatTimeDisplay is called how many times will pad be called?
 // =============> write your answer here
+// 3 times: once each for totalHours, remainingMinutes and remainingSeconds in the return line.
 
 // Call formatTimeDisplay with an input of 61, now answer the following:
+// With seconds = 61: remainingSeconds = 61 % 60 = 1, totalMinutes = (61 - 1) / 60 = 1,
+// remainingMinutes = 1 % 60 = 1, totalHours = (1 - 1) / 60 = 0.
+// formatTimeDisplay(61) returns "00:01:01".
 
 // b) What is the value assigned to num when pad is called for the first time?
 // =============> write your answer here
+// 0. The first call is pad(totalHours), and totalHours is 0.
 
 // c) What is the return value of pad when it is called for the first time?
 // =============> write your answer here
+// "00". num.toString() gives "0", which is 1 character long, so the while loop adds
+// one "0" to the front, making it 2 characters.
 
 // d) What is the value assigned to num when pad is called for the last time in this program?  Explain your answer
 // =============> write your answer here
+// 1. The last call is pad(remainingSeconds), and remainingSeconds is 61 % 60 = 1.
 
 // e) What is the return value of pad when it is called for the last time in this program?  Explain your answer
 // =============> write your answer here
+// "01". "1" has length 1, so the while loop adds one leading "0", then stops once the
+// length is 2.
+
+console.log(formatTimeDisplay(61)); // 00:01:01
\ No newline at end of file