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Proportion

Find the four...

Question

Find the fourth proportional.
15:20 :: 9:?

A

14

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B

12

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C

7

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D

3

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Solution

The correct option is B 12
Let the fourth proportional be x.
$⟹$ 15, 20, 9 and x are proportional.
i.e., 15:20 :: 9:x
$15 \times x = 20 \times 9 (P r o d u c t o f e x t r e m e s = P r o d u c t o f m e a n s)$
$⟹$ x = $\frac{20 \times 9}{15}$
$⟹$ x = 12
Hence the fourth proportional is 12.

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