lesson
Exchange before subtracting
1 · Learn
In column subtraction, keep equal places lined up. If there are too few ones, exchange one ten for ten ones. The total value stays the same.
For 452 − 128, exchange a ten: 452 becomes four hundreds, four tens and twelve ones. Now each column can be subtracted.
2 · Worked example
The hundreds give 4 − 1 = 3, so 452 − 128 = 324. I check 324 + 128 = 452.
3 · Your turn
Find 631 − 245 using exchanges.
Check your answer
386. Exchange a ten to get 11 ones; then exchange a hundred to get 12 tens. The column differences are 6 ones, 8 tens and 3 hundreds.
4 · Apply your learning
Model an exchange with drawn hundreds, tens and ones before using the written columns.
Adult guidance and safety
Use objects or a drawn model before relying on a rule. Explain each step, check the units and revisit with varied examples. An adult can read the instructions without supplying the answer.
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