Adding and subtracting (age 7+)
Add and subtract numbers with up to three digits using formal written methods of columnar addition and subtraction
What a learner can do afterwards
- Set out a columnar addition correctly with digits aligned by place value
- Carry out columnar subtraction with exchange (borrowing) when needed
- Check the answer using the inverse operation or estimation
1 · Read
You have already added and subtracted numbers up to 1000 by place value. Now you will set the work out neatly in columns. Write one number above the other so the digits line up by place value: ones under ones, tens under tens, hundreds under hundreds.
Add 362 + 259. Ones: 2 + 9 = 11, so write 1 and carry 1 ten. Tens: 6 + 5 + 1 = 12, so write 2 and carry 1 hundred. Hundreds: 3 + 2 + 1 = 6. The answer is 621.
Subtract 725 - 348 in columns. Ones: 5 is smaller than 8, so exchange 1 ten for 10 ones. Now 15 - 8 = 7. Tens: 1 is smaller than 4, so exchange 1 hundred for 10 tens. Now 11 - 4 = 7. Hundreds: 6 - 3 = 3. The answer is 377.
In every column, take the bottom digit away from the top digit. Never flip them round. If the top digit is smaller, exchange 1 from the next column to the left first, then subtract. Exchanging is also called borrowing.
Check an answer with the opposite operation. For 438 + 276 = 714, subtract 276 from 714 and get 438 again. You can also estimate: 438 is about 440 and 276 is about 280, so the sum is about 720. The exact answer 714 is close.
Line up digits by place value, start at the ones, carry or borrow as needed, then check your answer by working backwards or estimating.
Original estimation, checking and place-value text authored for the os-taxonomy project.
2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
10 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.