🚀 JavaScript Math Operators

Master Arithmetic in Code!

Grade 8 JavaScript Programming

🎯 Today's Objective

Students will use arithmetic operators to write JavaScript programs that solve problems with 80% accuracy.

By the end of class, you'll write programs that calculate costs, split bills, and animate graphics!

🍕 Warm-Up: The Mystery Restaurant Bill

Four friends went to dinner. The bill was $68.

They want to split it equally, BUT one friend has a $10 gift card.

How much does each person actually pay?

Think: What calculations do you need? In what order?

🎯 The Programming Pattern

1️⃣

GET

Collect information from the user using readInt() or readFloat()

2️⃣

CALCULATE

Use operators (+, −, *, /, %) to solve the problem

3️⃣

DISPLAY

Show results using console.log()

Every program you write will follow this pattern!

🔢 JavaScript Math Operators

+

Addition

Adds two numbers

5 + 3 = 8

−

Subtraction

Subtracts numbers

10 - 4 = 6

*

Multiplication

Multiplies numbers

6 * 7 = 42

/

Division

Divides numbers

20 / 4 = 5

🔄 The Mysterious Modulus Operator

What is % (Modulus)?

It finds the REMAINDER after division

17 % 5 = 2

Because 17 ÷ 5 = 3 remainder 2

💡 Use for: Checking odd/even, finding leftovers, time conversion

💻 Live Demo: Pizza Program

Problem: Calculate total cost for pizzas at $12 each

// STEP 1: GET information
let pricePerPizza = 12;
let quantity = readInt("How many pizzas? ");

// STEP 2: CALCULATE the total
let total = pricePerPizza * quantity;

// STEP 3: DISPLAY the result
console.log("Your total is $" + total);
                

⚠️ Common Mistake: Wrong Operator!

❌ WRONG

12 + 5 = 17

Using + gives you the wrong answer!

5 pizzas should NOT cost $17!

✅ CORRECT

12 * 5 = 60

Using * multiplies correctly!

5 pizzas at $12 each = $60

Choose the RIGHT operator or your program breaks!

🎯 How to Choose the Right Operator

Combining amounts? → Use + (total savings, combined scores)
Finding what's left? → Use − (change, discounts)
Repeated items? → Use * (price × quantity)
Splitting equally? → Use / (split bills, averages)
Finding remainder? → Use % (odd/even, leftovers)

🌐 Quia Interactive Practice

Before we code in CodeHS, let's see these operators in action!

https://www.quia.com/pages/jessica559si/javacriptmath

Watch how changing operators changes results completely

💰 CodeHS 2.5.3: Bookstore

Your Task:

Write a program for a bookstore that:

Asks how many books the customer wants
Calculates the total cost (books are $15 each)
Displays the total

Example: 3 books should cost $45

🐦 CodeHS 2.5.4: Flying Pigeon

Animation = Math Repeated Quickly!

To move something across the screen, you change its position using math:

// Move right: ADD to x position
pigeon.x = pigeon.x + 3;

// Move up: SUBTRACT from y position
pigeon.y = pigeon.y - 1;
                

Every 0.05 seconds, the computer recalculates position!

🐛 Debug Challenge: Find the Bugs!

// Movie ticket program - has 3 bugs!
let ticketPrice = 10
let numTickets = readInt("How many tickets? ");
let popcorn = 6;

let total = ticketPrice + numTickets + popcorn;
console.log("Your total is: " total);

let change = 50 / total;
console.log("Your change from $50 is: $" + change);
            

Can you find all 3 bugs? Think about operators and syntax!

🧩 Practice Problems

Problem 1: Gaming Console 🎮

Console costs $300. You save $15/week. Calculate weeks needed.

Problem 2: Road Trip 🚗

Car gets 28 mpg. Gas costs $3.50/gallon. Calculate cost for 280-mile trip.

💡 Pro Tips for Success!

✅ Choose the RIGHT operator - Using + instead of * breaks everything!
✅ Order matters - Use ( ) to control calculations
✅ Test your code - Try the example numbers first
✅ Add comments - Explain what each step does
✅ Variable names matter - Use totalCost not x

💭 Reflection

When would using the WRONG operator completely break your program?

Give a specific example from today.

Think-Write-Share with a partner

🏠 Homework Choice Board

Pick 2 of 4 activities:

Bill Splitter Pro 💵 - Split bills with tip options (15%, 18%, 20%)
Grade Calculator 📊 - Points earned ÷ points possible × 100
Time Converter ⏰ - Minutes to hours and minutes using / and %
Profit Calculator 📈 - Revenue − costs, profit margin percentage

BONUS: Complete Quia interactive activities for extra credit!

🎉 Great Job Today!

You learned:

The 5 arithmetic operators
The GET → CALCULATE → DISPLAY pattern
How to choose the right operator
How to debug code
How animation uses math!

Keep coding! 🚀