2
One long jump back
Learner’s Book 1: Unit 13, pp. 189–190
GoalSubtract by drawing one jump back, and read a number line to find the calculation.
New words
jump back · a hop to the left on the number lineland · the number where a jump endscalculation · a sum that we work out
Explanation
We can count back with one long jump instead of many small ones. To find 15 − 7, start at 15 and draw one jump of 7 to the left. The jump is labelled −7. It lands on 8, so 15 − 7 = 8. Both ways give the same answer. We can also read a number line. Look at where the jump starts, how far it goes and where it lands.
Worked examples
18 − 5: start at 18 and draw one jump of 5 to the left. It lands on 13. So 18 − 5 = 13.
20 − 9: start at 20 and draw one jump of 9 to the left. It lands on 11. So 20 − 9 = 11.
We can check a subtraction by adding. 15 − 7 = 8. Now add: 8 + 7 = 15. It matches, so the answer is right.
Class activity
Back jumps: use a number line to 20. Pick a start between 12 and 20 and a jump size between 2 and 9. Draw one jump back, write the subtraction, then check it by adding. Swap with a partner.
Practice
1
What is 16 − 5? Draw one jump back.
11
2
What is 13 − 6? Draw one jump back.
7
3
What subtraction does this number line show?
14 − 9 = 5
4
Sam starts at 17 and draws one jump back of 8. Where does he land?
9