Since you can count by tens, finding 10 more or 10 less is quick: no counting by ones needed.
Key idea
10 more changes the tens digit up by 1. 10 less changes it down by 1. The ones digit stays the same.
Why it works
Adding 10 adds one ten-stick. The loose ones do not change, so only the tens digit moves.
Worked example: 10 more and 10 less
What is 10 more than 34? What is 10 less than 34?
- 34 is 3 tens and 4 ones.
- 10 more: 4 tens and 4 ones is 44.
- 10 less: 2 tens and 4 ones is 24.
Answer: 44 and 24
Common mistake
Changing the ones digit and saying 10 more than 34 is 35. That is only 1 more. On the hundred chart, 10 more is one row down.
For grown-ups: Play “10 more, 10 less” with the hundred chart or with house numbers on your street.