Find the interest rate, given the increase over the period of 9 months Goods were bought for Rs. 600 and sold the same for Rs. 688.50 at a credit of 9 months and thus gaining 2% The rate of interest per annum is
A.$16\ ^2/_3\ \%$ B. $14\ ^1/_2\ \%$ C. $13\ ^1/_3\ \%$ D. $15\ \%$
First, please explain me the question, I didn't get the thus gaining 2% part.
 A: Selling Price
$$S.P.=102\%\times600=612 $$ 
Present Worth$$P.W.=612$$ and Sum$$S=688.5$$
True Discount$$T.D.=688.5-612=76.5$$
Thus, Simple Interest on $Rs. 612$ for $9$ months is $Rs. 76.50$
Therefore, Rate
$$R=\frac{100\times76.5}{612\times\frac34}=16\frac23\%$$
A: $\newcommand{\angles}[1]{\left\langle\, #1 \,\right\rangle}
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$$
\mbox{Every month you pay}\quad
600\,{r/1200 \over 1 - \pars{1 + r/1200}^{-9}} = {688.50 \over 9}
$$

$$
\mbox{From this equation, we'll get}\
\approx 34.13\ \%\ \mbox{per annum}.
$$

