A person bought some pens at the rate of 30 per rupee and sold them at the rate of 20 for a rupee. What is the profit percentage?

50%

Cost price of 1 pen = 1/30

Selling price of 1 pen = 1/20

Profit = $\frac{1}{20} - \frac{1}{30}$

         =$\frac{1}{60}$ 

Profit percent = $\frac{profit}{Cost\; price}$ x 100

                     = $\frac{\frac{1}{60}}{\frac{1}{30}}$ x 100 = 50%