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Checking your results by doing an equivalent operation is a valuable skill to learn.

Look carefully at the following to understand the method used when multiplying by numbers such as 9, 99 or 999.

 11 × 9 = 11 × (10 - 1) = 110 - 11 = 99

521 × 9 = 521 × (10 - 1) = 5210 - 521 = 4689

19 × 99 = 19 × (100 - 1) = 1900 - 19 = 1881

17 × 999 = 17 × (1000 - 1) = 17000 - 17 = 16983

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