Subtraction strategies
Lesson 8 of 12
Column subtraction with exchange
Objective. Subtract three-digit numbers using exchange between columns.
The key idea
When the top digit is too small, exchange one from the column to its left: one ten becomes ten ones, one hundred becomes ten tens.
Worked example
Work out 402 − 137.
- 1Ones: 2 − 7 is not possible. Exchange from the tens, but tens are 0, so exchange 1 hundred into 10 tens first.
- 2Now 3 hundreds, 10 tens, 2 ones. Exchange 1 ten: 3 hundreds, 9 tens, 12 ones.
- 312 − 7 = 5, 9 − 3 = 6, 3 − 1 = 2.
Answer: 265
Practice
Each question comes with a picture and a listen button. Try it first, then reveal the answer.
Common mistake. Subtracting the smaller digit from the larger one in each column regardless of which is on top.
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