In Grade 1 you found 10 More and 10 Less for two-digit numbers. With hundreds you can also find 100 more and 100 less, in your head.
Why it works
10 more means one more ten-rod, so only the tens digit changes. 100 more means one more hundred-flat, so only the hundreds digit changes. The other blocks did not move, so their digits stay the same.
Worked example: 10 more
What is 10 more than 365?
- One more ten: 6 tens become 7 tens.
- 365 becomes 375.
Answer: 375
Worked example: 100 less
What is 100 less than 412?
- One less hundred: 4 hundreds become 3 hundreds.
- 412 becomes 312.
Answer: 312
Worked example: Crossing a hundred
What is 10 more than 395?
- 395 has 9 tens. One more makes 10 tens.
- 10 tens make 1 hundred, so the hundreds go from 3 to 4 and the tens become 0.
- 395 becomes 405.
Answer: 405
Key idea
To add or take away 10, change the tens digit by 1. To add or take away 100, change the hundreds digit by 1.
Common mistake
Writing 10 more than 395 as 3105. The tens place can’t hold 10: ten tens make a new hundred, so the answer is 405. Check with blocks: 39 tens and 1 more ten is 40 tens, which is 400, plus 5.
Grown-up tip: Play “10 more, 100 less” with house numbers on a walk.