3
Number families
Discover Mathematics: Chapter 6
GoalUse three numbers to make adding and taking-away sentences, and use counting on to find answers.
New words
fact family · three numbers that go togetherpart · one of the small groupswhole · all the parts togethercount on · start at a number and count up
Explanation
Three numbers can make a family. Two numbers are the parts and one number is the whole. Look at the cherries: 3 and 4 are the parts, and 7 is the whole. This family gives four sentences: 3 + 4 = 7, 4 + 3 = 7, 7 − 3 = 4 and 7 − 4 = 3. To add 5 + 2, start at 5 and count on two more: 6, 7.
Worked examples
Start at 5 birds and count on 2 more: 6, 7. So 5 + 2 = 7.
The family 2, 6, 8: 2 + 6 = 8, 6 + 2 = 8, 8 − 2 = 6 and 8 − 6 = 2.
Take 4 away from 9 and 5 are left. So 9 − 4 = 5. The family 4, 5, 9 also gives 9 − 5 = 4.
Class activity
Family cards. Write three numbers of a family, such as 3, 5 and 8, on a paper. The child tells you four sentences using them: two adding and two taking away. Use buttons to check each sentence. Try the families 2, 4, 6 and 4, 5, 9.
Practice
1
Complete: 3 + 4 = __ and 4 + 3 = __.
7 and 7
2
What is 8 − 3? Count what is not crossed out.
5
3
Start at 5 and count on 2. What do you reach?
7
4
The family is 2, 6 and 8. Write the answer: 8 − 2 = __.
6
5
Tamer has 4 red and 5 yellow balloons. How many in all? Which two take-away sentences go with this family?
9 balloons. 9 − 4 = 5 and 9 − 5 = 4.