3
Take away in two steps
Learner’s Book 1: Unit 13, pp. 191–192
GoalSplit the number you take away, or go back to 10 first, to subtract in two steps.
New words
decompose · split a number into smaller partscompose · put smaller parts together to make a numberstep · one jump in a series of jumps
Explanation
Sometimes it is easier to take away in two steps. To find 17 − 12, split 12 into 10 and 2. First jump back 10, from 17 to 7. Then jump back 2, from 7 to 5. So 17 − 12 = 5. We can also jump back to 10 first. To find 15 − 8, jump back 5 to reach 10. Then jump back 3 more, because 5 and 3 make 8. So 15 − 8 = 7. To check, add the two jumps: 5 + 3 = 8.
Worked examples
17 − 12: split 12 into 10 and 2. Jump back 10 to 7, then jump back 2 to 5. So 17 − 12 = 5.
13 − 5: go back to 10 first. 3 gets us to 10. Then 2 more, because 3 and 2 make 5. We land on 8. So 13 − 5 = 8.
16 − 9: jump back 6 to reach 10, then jump back 3 more. 6 and 3 make 9. We land on 7. So 16 − 9 = 7.
Class activity
Two-step race: write 6 subtractions with a start from 12 to 19 and a take-away number from 5 to 12. Solve each one by jumping back in two steps on a number line. Check each one by adding the jumps.
Practice
1
What is 16 − 12? The first jump back of 10 is drawn. Now jump back 2 more. Where do you land?
4, so 16 − 12 = 4.
2
What is 14 − 9? The first jump back of 4 reaches 10. How many more do you jump back, and where do you land?
5 more, because 4 and 5 make 9. You land on 5, so 14 − 9 = 5.
3
What is 12 − 5? The first jump back of 2 reaches 10. Jump back the rest of the 5. Where do you land?
Jump back 3 more to 7, so 12 − 5 = 7.
4
To take away 13, we split it into 10 and what?
3