Addition, subtraction & money
Subtraction
In this unit you subtract a 1-, 2- or 3-digit number from a 3-digit number. You learn what to do when you don't have enough ones to subtract from, and how to estimate first.
Line up the places
Write the numbers so the hundreds, tens and ones line up in columns, with the larger number on top. Start subtracting from the ones (the right-hand column), then the tens, then the hundreds.
Regrouping to get enough ones
Sometimes the top number has too few ones to subtract from. Exchange 1 ten for 10 ones: cross out a ten and add 10 to the ones. Now you have enough ones to subtract. Example: for 342 - 6, 2 ones is not enough, so regroup: 12 ones - 6 = 6.
Estimate first
Round each number to the nearest 10, then subtract the rounded numbers. This gives an estimate to check your answer. Example: 178 - 25 → 180 - 30 = 150, so the real answer should be close to 150.
Worked examples
Subtract a single digit from a 3-digit number
- Look at the ones: 5 - 9. There are not enough ones.
- Regroup 1 ten into 10 ones, so the ones become 15.
- 15 - 9 = 6. Bring down the tens and hundreds.
Answer: 375 - 9 = 366
Estimate and subtract a 2-digit number from a 3-digit number
- Estimate: 178 rounds to 180, 25 rounds to 30, so 180 - 30 = 150.
- Subtract the ones: 8 - 5 = 3.
- Subtract the tens: 7 - 2 = 5. Bring down the 1 hundred.
Answer: 178 - 25 = 153 (close to the estimate of 150)
Estimate and subtract a 3-digit number from a 3-digit number
- Estimate: 472 rounds to 470, 267 rounds to 270, so 470 - 270 = 200.
- Ones: 2 - 7 is not enough, so regroup 1 ten: 12 - 7 = 5.
- Tens: after regrouping, 6 - 6 = 0. Hundreds: 4 - 2 = 2.
Answer: 472 - 267 = 205 (close to the estimate of 200)
Where you see this in real life
- Finding how many milk cartons a store can sell after some are damaged.
- Working out how many parcels are left at the post office after a van takes some away.
- Finding how many children are still at the museum after some return to school.
Key words
- Subtract
- To take one number away from another.
- Regroup
- To exchange 1 ten for 10 ones so you have enough ones to subtract.
- Exchange
- To swap 1 of one place for 10 of the place below, such as 1 ten for 10 ones.
- Difference
- The answer to a subtraction — what is left after you take away.
- Estimate
- A close answer found by rounding, used to check your calculation.