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If a, b, c are three numbers such that a:b = 5:9 and b:c = 27:35, find a:c.

  • A.3:7
  • B.7:3
  • C.4:7
  • D.7:4

Answer: A

The ratio a:c is 3:7.

Make the b terms equal. In the first ratio b is 9 and in the second 27, so multiply the first ratio by 3:

\[ \begin{aligned} a : b &= 5 : 9 = 15 : 27 \\ b : c &= 27 : 35 \\ a : c &= 15 : 35 = 3 : 7 \end{aligned} \]

7:3 inverts the ratio, and 4:7 and 7:4 do not follow from the given ratios.

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