Decimals in Action: Adding and Subtracting

Decimals in Action: Adding and Subtracting

Years 7-8 Mathematics New Zealand Curriculum 45-minute lesson

Learning Objectives

Add and subtract decimals to at least two decimal places accurately Use estimation to verify decimal calculations Apply decimal operations to solve real-world problems Explain decimal relationships and reasoning clearly

Quick Recap Challenge

What is the place value of the digit 7 in 45.37? Which is larger: 2.8 or 2.08? Round 6.847 to one decimal place

Adding Decimals: The Method

Line up the decimal points vertically Add zeros as placeholders if needed Add column by column from right to left Keep the decimal point in the same position

Estimation First!

Before calculating 4.6 + 3.8, estimate the answer Round to whole numbers: 5 + 4 = 9 Now calculate exactly: 4.6 + 3.8 = 8.4 Is your answer reasonable? Yes, close to 9!

Subtracting Decimals

Line up decimal points (same as addition) Add zeros as placeholders when needed Subtract column by column from right to left Borrow from the next column when necessary

Real-World Shopping Challenge

You have $20.00 to spend Item 1: Sandwich $6.75 Item 2: Drink $2.45 Item 3: Snack $3.25 Calculate total cost and change

Think-Pair-Share Discussion

{"left":"What happens when you add a larger decimal to a smaller one?\nHow does estimation help check your answers?","right":"Why is it important to line up decimal points?\nWhat mistakes do people commonly make with decimals?"}

Common Decimal Mistakes to Avoid

Independent Practice Time

Complete worksheet problems 1-12 Include both horizontal and vertical formats Show your estimation work Check answers using a calculator if needed

Quick Check Quiz

Calculate: 15.6 + 8.75 Estimate first: 16 + 9 = 25 Exact answer: 24.35 Is this reasonable? Yes!

Lesson Summary & Next Steps

We learned to add and subtract decimals accurately Estimation helps us check if answers are reasonable Lining up decimal points is crucial for success Next: We'll explore multiplying and dividing decimals