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: at what rate percent per annum will a sum of money double in 6 years . 
Options: a ) 12.5 % , b ) 13.5 % , c ) 11.5 % , d ) 14.5 % , e ) 16.6 %
Output:
e