Method H - Complementary Method

CPD for Primary: Subtraction Methods for KS2 Maths (by Jo Morgan), topic 2.1.8

Complementary Method: 753 − 491

Find the complement of the subtrahend and add it to the minuend. The complement is the number we add to the subtrahend to get it to the nearest power of 10.

Step 1

Here we find the complement of 491 (i.e. what we add to 491 to make 1000). We can instantly write down 509 using the method described for calculating complements.
Now we add the complement of the subtrahend to the minuend.

Step 2

And finally, we subtract 1000 from the difference.

Breaking down the maths

Here, we have worked out 753 + (1000 − 491) − 1000.
Simplifying, we get 753 + (1000 − 491) − 1000 = 753 − 491 as required.

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