A man buys 13 pens for a rupee. How many pens should he sell for a rupee to earn a profit of 30%?

10

If a man buys 13 pens for a rupee,

CP of 1 pen = $\frac{1}{13}$

For the man to earn profit of 30%,

If CP = 100, profit = 30

SP = 100 + 30 = 130

The ratio of CP to SP will be 100 : 130

Here, C.P. = 100R = $\frac{1}{13}$

Hence,

SP = 130R = 130 x  $\frac{1}{13 \times 100}$ =  $\frac{1}{10}$

Thus, a total of 10 pens should be sold to earn a profit of 30%.