Video: Multiplying by Breaking Numbers Apart and Using the Associative Property

Kerrie Lynskey

Which of these is equal to 18 × 5? Hint: remember that you can replace one of the factors by an expression it is equal to. [A] 10 × 8 × 5 [B] 9 × 9 × 5 [C] 8 × 2 × 5 [D] 9 × 2 × 5.

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Video Transcript

Which of these is equal to 18 times five? Hint: remember that you can replace one of the factors by an expression it is equal to.

10 times eight times five, nine times nine times five, eight times two times five, nine times two times five.

So which of these is equal to 18 times five. And the hint tells us that we can replace one of the factors by an expression it is equal to. The factors in this case are 18 and five. We have to replace one of the factors. It couldn’t be five because all of these expressions already have five in them. So it must be 18. So we need to replace 18 by an expression that it’s equal to.

10 times eight is 80: that’s not equal to our factor 18. Nine times nine is 81: that’s not equal either. Eight times two is 16: so that’s not equal to 18 either. But nine times two is equal to 18. We can replace the factor 18 with the expression nine times two and then times that by five. So nine times two times five is equal to 18 times five.

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