# algebra 1

I am working on cross products of proportions. one question is 5/7 = 3w/21
Can someone help?

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1. 21 w = 105 is the next step if you match the "cross products" of numerator-denominator pairs.

Then divide each side by 21 to get w.

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posted by drwls
2. multiply both sides of this equation by 21
(5/7) 21 = (3w/21) 21
5*3 = 3 w
now divide both sides by 3
5 = w

(I do not like the cross multiplication language. It is safer to multiply both sides by a common multiple of the denominators, which is really what cross multiplication means.)
If you did it with cross multipication you would have
5 * 21 = 7 * 3 * w
5 * 21 = 21 * w
5 = w

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posted by Damon

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