Topic 5 of 8

Adding and subtracting within 1,000

Step 4 · Place value to 1,000 topics

Add and subtract three-digit numbers by working with hundreds, tens and ones, regrouping when a place has too many or too few, and find 10 or 100 more or less.

Adding hundreds, tens, and ones

Add like places together: hundreds with hundreds, tens with tens, ones with ones. 243 + 325: 200 + 300 = 500, 40 + 20 = 60, 3 + 5 = 8. So 568.

243 + 325 = 500 + 60 + 8 = 568

Regrouping

When a place has 10 or more, regroup: 10 ones make a ten, and 10 tens make a hundred.

Example: Regrouping when adding

Work out 368 + 257.

Hundreds: 300 + 200 = 500.
Tens: 60 + 50 = 110.
Ones: 8 + 7 = 15.
Put them together: 500 + 110 + 15 = 625.

Answer625

368

257

368 and 257: 5 hundreds, 11 tens and 15 ones. Regroup to get 6 hundreds, 2 tens and 5 ones: 625.

When subtracting, a place may not have enough. Break one of the next place up: a ten into 10 ones, or a hundred into 10 tens.

Example: Regrouping when subtracting

Work out 524 − 158.

Ones: 4 is not enough to take 8. Break a ten: 4 + 10 = 14 ones; 14 − 8 = 6.
Tens: 1 ten is left, not enough to take 5. Break a hundred: 1 + 10 = 11 tens; 11 − 5 = 6.
Hundreds: 4 left; 4 − 1 = 3.

Answer366

Check by adding back: 366 + 158 = 524.

10 or 100 more or less

Adding 10 changes the tens digit; adding 100 changes the hundreds digit. 10 more than 462 is 472; 100 more is 562. 10 less is 452; 100 less is 362.

100 less	10 less	Number	10 more	100 more
362	452	462	472	562
208	298	308	318	408

Try it yourself

Work each one on paper first, then open the answer to check.

412 + 235
Show answer
647.
456 + 278
Show answer
734.
785 − 342
Show answer
443.
603 − 245
Show answer
358.
What is 100 more than 690?
Show answer
790.
What is 10 less than 702?
Show answer
692.