Task: Question Answering
Input (English): Problem: if @ is a binary operation defined as the difference between an integer n and the product of n and 5 , then what is the largest positive integer n such that the outcome of the binary operation of n is less than 21 ? 
Options: a ) 1 , b ) 2 , c ) 3 , d ) 4 , e ) 5
Output (English): e