By selling 99 pens, a trader gains the cost of 33 pens. Find his gain percentage?

A. 33 1/3%
B. 50%
C. 25%
D. 66 2/3%
E. None of these
Explanation:
Let the CP of each pen be Rs. 1.
CP of 99 pens = Rs. 99
Profit = Cost of 33 pens = Rs. 33
Profit% = 33/99 * 100 = 33 1/3%