If 3 : x :: 9 : 15, find x.

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A 5
B 10
C 15
D 20

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Answer: A. 5

In a proportion the ratios are equal, so 3/x = 9/15. Cross-multiplying gives 9x = 3 x 15 = 45, so x = 45 / 9 = 5. Check: 3 : 5 and 9 : 15 both simplify to 3 : 5.

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If 3 : x :: 9 : 15, find x.

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