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How did they come up with 24 to the following ?

Calculate the 4th proportional to 5,8,15 ?

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4 can't be the solution. – Aang Jul 3 '12 at 6:32
Sorry that was a misprint - I meant 24 – MistyD Jul 3 '12 at 6:33
$\frac{5}{8}=\frac{15}{x}$ which gives you $24$ – Saurabh Jul 3 '12 at 6:33
Agree with @SaurabhHota. – Aang Jul 3 '12 at 6:37
@SaurabhHota Thanks ..That was simple – MistyD Jul 3 '12 at 6:40

$5:8 :: 15:x$ implies $5x = 15\times8$.
Then you will get the answer.

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