A man bought oranges at the rate of 8 for Rs. 34 and sold them at the rate of 12 for Rs. 57. How many oranges should be sold to earn a net profit of Rs. 45 ?

CP of 1 orange $$ = {\text{Rs}}{\text{.}}\,\frac{{34}}{8} = {\text{Rs}}{\text{.}}\,4.25$$
SP of 1 orange $$ = {\text{Rs}}{\text{.}}\,\frac{{57}}{{12}} = {\text{Rs}}{\text{.}}\,4.75$$
Profit on each apple = (4.75 - 4.25) = Rs. 0.50
∴ Number of apples required $$ = \frac{{45}}{{0.50}} = 90$$