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
The lesson
You can already add and subtract numbers up to 1000 in your head or on a number line. Now you will learn to write it down neatly using 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 347 + 265. Ones: 7 + 5 = 12, so write 2 and carry 1 ten. Tens: 4 + 6 + 1 carried = 11, so write 1 and carry 1 hundred. Hundreds: 3 + 2 + 1 carried = 6. The answer is 612.
Subtract 512 − 278. Ones: 2 is smaller than 8, so exchange 1 ten to make 12. 12 − 8 = 4. Tens: the tens digit is now 0, smaller than 7, so exchange 1 hundred to make 10. 10 − 7 = 3. Hundreds: 4 − 2 = 2. The answer is 234.
Always check your answer. For addition, subtract one number back from the total: 612 − 265 should give 347 again. Or estimate first by rounding: 350 + 270 is about 620, close to 612, so the exact answer feels right.
Line up digits by place value, start at the ones, carry or borrow as needed, then check your answer by working backwards or estimating.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 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.