Oranges are bought at 5 for Rs. 10 and sold at 6 for Rs. 15. The gain percentage is = ?

Let the (LCM of 5 and 6 = 30) oranges be bought.
$$\eqalign{ & \therefore {\text{CP of 30 oranges}} \cr & = \frac{{10}}{5} \times {\text{30}} \cr & = {\text{Rs}}{\text{. 60}} \cr & {\text{Their Selling price}} \cr & = \frac{{15}}{6} \times 30 \cr & = {\text{Rs}}{\text{. 75}} \cr & {\text{Profit}} = {\text{Rs}}{\text{.}}\left( {{\text{75}} - {\text{60}}} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{. 15}} \cr & \therefore {\text{Profit percent}} \cr & = \left( {\frac{{15}}{{60}}} \right) \times 100 \cr & = 25\% \cr} $$