2
Missing addends
Discover Mathematics P2: Term 1, Lessons 11–20
GoalFind the missing number in an addition sentence like 6 + ? = 12.
New words
addend · a number that we addmissing · not there yet, the one we must findtotal · all together
Explanation
Sometimes one addend is missing: 6 + ? = 12. We know the two parts must make the total 12. There are two easy ways. Count on from 6 to 12 and count the steps: that is 6. Or take away: 12 − 6 = 6. In a story, ask yourself: how many more do we need to reach the total?
Worked examples
6 + ? = 12. Count on from 6 to 12: 7, 8, 9, 10, 11, 12. That is 6 steps. The missing addend is 6.
At 8 p.m. Omar saw 3 stars. At 9 p.m. he saw 13 stars. How many stars came out? 3 + ? = 13, so 13 − 3 = 10 stars.
Aya had 20 sweets. After lunch she has 11 left. 20 − ? = 11. Count up from 11 to 20: that is 9. She ate 9 sweets.
Class activity
Hidden number game. Put 10 spoons on the table and cover some with a cloth. Ask: I can see 4. How many are hidden? Say 4 + ? = 10 and find the answer. Then swap.
Practice
1
Find the missing number: 7 + ? = 15.
8
2
Find the missing number: 10 + ? = 16.
6
3
Find the missing number: 9 + ? = 19.
10
4
Find the missing number: 14 − ? = 9.
5 (9 + 5 = 14)
5
Hana has 8 stickers. She wants 15. How many more does she need?
7 (8 + 7 = 15)