A shopkeeper purchased 70 kg of potatoes for Rs. 420 and sold the whole lot at the rate of Rs. 6.50 per kg. What will be his gain percent ?

$$\eqalign{ & {\text{C}}{\text{.P}}{\text{. of 1 kg}} \cr & = {\text{Rs}}.\left( {\frac{{420}}{{70}}} \right) \cr & = {\text{Rs}}{\text{. }}6. \cr & {\text{S}}{\text{.P}}{\text{. of 1 kg}} \cr & = {\text{Rs}}{\text{. }}6.50. \cr & \therefore {\text{Gain}}\% = \left( {\frac{{0.50}}{6} \times 100} \right)\% \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{25}}{3}\% \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 8\frac{1}{3}\% \cr} $$