Adding tens
345 is 3 hundreds, 4 tens and 5 ones. Adding 20 adds 2 tens and nothing else, so only the tens change: 4 tens and 2 tens make 6 tens.
The hundreds and the ones stay as they were. So 345 + 20 = 365, and the only digit that changed is the tens digit, from 4 to 6.
345 is 3 hundreds, 4 tens and 5 ones.
Add 2 tens and there are 6 tens. The 3 hundreds and the 5 ones have not changed: 365.
Counting on in tens from 345: 355, then 365. The ones digit is 5 at every step.
Adding hundreds
Adding hundreds is the same move in the hundreds place. 200 is 2 hundreds, so 345 + 200 changes only the hundreds digit: 3 hundreds and 2 hundreds make 5 hundreds. So 345 + 200 = 545.
Put the 2 in the right place. Adding it to the tens instead gives 365, but 200 is 2 hundreds, not 2 tens.
345 + 200: 5 hundreds, with the same 4 tens and 5 ones. That is 545.
Taking away
Subtracting tens or hundreds changes one digit in the same way. For 345 − 200, 3 hundreds take away 2 hundreds leaves 1 hundred, so 345 − 200 = 145. For 345 − 20, 4 tens take away 2 tens leaves 2 tens, so 345 − 20 = 325.
345 − 200: 1 hundred is left, with the same 4 tens and 5 ones. That is 145.
When the tens go past 9
Sometimes the tens digit cannot simply change. 380 + 50 would need 8 tens and 5 tens, which is 13 tens, but a place holds only one digit. 13 tens are 1 hundred and 3 tens, so the hundreds digit goes up by 1 as well: 380 + 50 = 430.
A number line shows it. From 380, 20 reach 400, and the other 30 of the 50 take it on to 430.
380 + 50 goes past 400: a hop of 20 reaches 400, and a hop of 30 lands on 430.
Worked example: Points Won and Lost on a Score Board
Question Kai has 430 points on the score board. He wins 200 points in the next round. Then he loses 50 points. (a) How many points does Kai have after he wins 200? (b) How many points does he have after he loses 50?
1.200 is 2 hundreds and 430 has 4 hundreds. 4 hundreds and 2 hundreds make 6 hundreds. The tens and the ones stay the same.
4 hundreds and 2 hundreds make 6 hundreds. The tens and the ones stay the same. 2.(a) 430 + 200 = 630, so Kai has 630 points after he wins.
(a) 430 + 200 = 630 points. 3.50 is 5 tens and 630 is 63 tens. 63 tens take away 5 tens is 58 tens.
630 is 63 tens, and 63 tens take away 5 tens is 58 tens. 4.(b) 630 − 50 = 580, so Kai has 580 points after he loses. Check: 580 + 50 = 630.
(b) 630 − 50 = 580 points.
Answer: (a) 630 points; (b) 580 points
Common mistakes
- Adding the 2 of 200 to the tens digit and writing 450. 200 is 2 hundreds, so the hundreds digit changes from 4 to 6.
- Taking 3 tens from 5 tens and writing 620. The 5 tens are being taken away from 63 tens, and 63 − 5 = 58, so the answer is 580.
More adding and subtracting in your head problems, worked step by step →