a) 15%
b) 10%
c) 20%
d) 5%

Answer: b
Explanation: Let the sum be Rs. P and rate of interest be R% per annum. Then,
$$\eqalign{ & P{\left( {1 + \frac{R}{{100}}} \right)^2} = 9680\,…..\,\left( 1 \right) \cr & P{\left( {1 + \frac{R}{{100}}} \right)^3} = 10648\,…..\,\left( 2 \right) \cr} $$
On dividing equation (2) by (1) :
$$\eqalign{ & 1 + \frac{R}{{100}} = \frac{{10648}}{{9680}} \cr & \Rightarrow \frac{R}{{100}} = \frac{{10648}}{{9680}} – 1 \cr & \Rightarrow \frac{R}{{100}} = \frac{{10648 – 9680}}{{9680}} \cr & \Rightarrow \frac{R}{{100}} = \frac{{968}}{{9680}} \cr & \Rightarrow \frac{R}{{100}} = \frac{1}{{10}} \cr & \Rightarrow R = \frac{1}{{10}} \times 100 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\, = 10\% \cr} $$