Instructions: In this task, you need to answer the given multiple-choice question on the gain. Gain is the value by which to multiply the input. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.
Input: Problem: by selling 100 pens , a trader gains the cost of 40 pens . find his gain percentage ? 
Options: a ) 40 % , b ) 50 % , c ) 60 % , d ) 70 % , e ) 30 %
Output:
a