3
Taking away to twenty
Discover Mathematics: Chapter 7
GoalTake away within 20, use number families, and read a simple day plan.
New words
count back · start at a number and count downfact family · three numbers that go togetherschedule · a list of what we do and whenleft · what stays after taking away
Explanation
To take away within 20, we can count back. For 14 − 6, start at 14 and count back 6: 13, 12, 11, 10, 9, 8. So 14 − 6 = 8. We can also use the picture: 14 stars with 6 crossed out leave 8. A fact family for 6, 8 and 14 has four sentences: 6 + 8 = 14, 8 + 6 = 14, 14 − 6 = 8 and 14 − 8 = 6. A schedule is a list of what we do and when.
Worked examples
Hamza has 15 fish. He gives 7 away. 15 − 7 = 8, so 8 fish are left.
The family 5, 9 and 14: 9 + 5 = 14, 5 + 9 = 14, 14 − 5 = 9 and 14 − 9 = 5.
Hana's schedule: she wakes up at 6 o’clock, has breakfast at 7 o’clock and goes to school at 8 o’clock.
Class activity
Write a day plan. Help the child write a short plan of a weekend day with three things and the o’clock time of each, for example wake up, lunch and bedtime. Draw a small clock beside each. Then read it aloud together.
Practice
1
What is 12 − 5? Count what is not crossed out.
7
2
What is 18 − 9?
9
3
Count back 3 from 13. Where do you stop?
10
4
The family is 7, 9 and 16. Write the answer: 16 − 7 = __.
9
5
Marwan has lunch at 2 o’clock and football at 4 o’clock. Which comes first?
Lunch