# direct proportional

Mr Tan’s monthly savings (S) is directly proportional to the square root of his monthly income (I). His income in January and February 2011 is 3600 and 2500 respectively. His savings in January is 80 more than in February. Find the amount he saved in January 2011.

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Please show your attempts to the problem to get good responses from people. –  math Jun 24 '13 at 11:58

$S_{Jan}$ $-$ January savings;
$S_{Feb}$ $-$ February savings;

main rule: $S = k \cdot \sqrt{I}$, $\quad$where $k$ $-$ constant value.

$S_{Jan} = k \cdot \sqrt{3600} = k\cdot 60$,
$S_{Feb} = k \cdot \sqrt{2500} = k \cdot 50$.

$S_{Jan}-S_{Feb}=80$.

$\cdots$

It remains to find out value $k$. For you.

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