Making up to a round number
You learn how much to add or take away to reach a round ten, hundred or thousand.
How to work it out
- Look at the last digit of the number.
- Going up: make the ones up to ten.
- Going down: take the ones away.
- Going to 100 or 1000? First jump to the next ten or hundred.
A worked example
62 + _ = 100
- Jump to the next ten: 62 + 8 = 70.
- Jump to 100: 70 + 30 = 100.
- Add the jumps: 8 + 30 = 38.
More examples
76 + _ = 80
- Look at the ones: 6.
- Make them up to ten: 6 + 4 = 10.
- So 76 + 4 = 80.
56 − _ = 50
- Look at the ones: 6.
- Take them away.
- So 56 − 6 = 50.
375 + _ = 1000
- Jump to the next hundred: 375 + 25 = 400.
- Jump to 1000: 400 + 600 = 1000.
- Add the jumps: 25 + 600 = 625.
Try it yourself
27 + • = 30
42 + • = 50
Watch out
Going up, you give the ones as the answer: for 76 + _ = 80 you say 6.
Going up, you need the number bond to ten: 6 goes with 4. Only going down do you take the ones themselves away.
You make both digits up to ten: for 62 + _ = 100 you say 48.
Check: 62 + 48 = 110, which is too much. Jump to 70 first, then you only need 3 more tens.
Tips
- The round number you want is the very next one: after 62 comes 70, after 375 comes 400.
Exceptions
- If the number already ends in 0, like 70, it is already a round ten and you do not need to add anything.
Why it works
To go from 76 to 80, you only look at the 6: 6 and 4 make ten. Those are the number bonds to ten you already know. From 62 to 100 is a big jump, but from 62 to 70 and then from 70 to 100 are two small jumps you can do. Add the jumps together, and you know how much you needed in total.