1
Hop back on the number line
Discover Mathematics P2: Term 2, Lessons 91–100
GoalSubtract by hopping back on a number line, in tens and ones.
New words
hop back · move to a smaller number on the number linedifference · the answer to a subtractionsubtract · take one number away from another
Explanation
To subtract on a number line, start at the first number and hop back. For 17 − 5, start at 17 and hop back 5 steps. You land on 12, so 17 − 5 = 12. For bigger numbers use bigger hops. For 38 − 15, hop back 10 to 28, then hop back 5 to 23. So 38 − 15 = 23. Sometimes it helps to land on a ten first. For 52 − 9, hop back 2 to 50, and then hop back 7 to 43. So 52 − 9 = 43. The place where you finish is the difference.
Worked examples
17 − 5: start at 17 and hop back 5. You land on 12.
38 − 15: hop back 10, then hop back 5. You land on 23.
52 − 9: hop back 2 to reach 50, then hop back 7. You land on 43.
Class activity
Number line on the floor. Draw a number line from 0 to 30 with chalk, or lay paper numbers on the floor. Stand on a number such as 25 and hop back the steps called by a partner, for example 4, then 10. Say where you land and write the subtraction.
Practice
1
Use the number line to find 15 − 9.
Hop back 9 from 15. You land on 6.
2
Find 18 − 12 on a number line.
Hop back 10 to 8, then hop back 2 to 6. So 18 − 12 = 6.
3
Find 37 − 16.
37 − 10 = 27 and 27 − 6 = 21. So the difference is 21.
4
Find 28 − 8. Where do you land?
You land on 20, so 28 − 8 = 20.
5
Karim says 52 − 9 = 41. Use hops to check.
52 − 2 = 50 and 50 − 7 = 43. So Karim is wrong, the answer is 43.