Egypt Primary 2 Mathematics, Week 7: Swap the addends

Ordering numbers and adding in any order

Week 7 · Ordering numbers and adding in any order · 3 lessons

Swap the addends

Discover Mathematics P2: Term 1, Lessons 21–30

GoalSee that adding in any order gives the same sum, and use it to add faster.

New words

addends · the numbers we addswap · to change placessum · the answer when we add

Explanation

When we add, we can swap the addends and the sum stays the same. 3 + 5 = 8 and 5 + 3 = 8. This helps us add fast: put the bigger number first and count on. For 2 + 16, swap to 16 + 2 and count on 2: 17, 18. This works for adding only. In subtraction the order matters: 5 − 3 is not 3 − 5.

Worked examples

3 balls and 5 balls make 8 balls: 3 + 5 = 8.

Swap the groups: 5 balls and 3 balls still make 8. 5 + 3 = 8.

2 + 16 is hard to count on from 2. Swap it: 16 + 2 = 18.

Class activity

Domino swap. Look at dominoes or two dice. Say the sum, then turn the domino around and say the sum again. Do you get the same sum? Why?

Practice

Solve 7 + 12. Then swap the addends and solve again.

Fill in the missing number: 9 + 6 = 6 + ?

Use a swap to solve 3 + 14.

Is 10 − 4 the same as 4 − 10? Why?

Hana has 8 stickers and Seif has 9. Write the sum two ways.