By selling 100 pens, a trader gains the cost of 40 pens. Find his gain percentage? Choose the correct choice from the following choices:
A) 40 % B) 50 % C) 60 % D) 70 % E) 30 %

Rationale: "let the cp of each pen be rs . 1 . cp of 100 pens = rs . 100 profit = cost of 40 pens = rs . 40 profit % = 40 / 100 * 100 = 40 % answer : a"
Answer: A