Task: Question Answering
Input (English): Problem: a man can do a piece of work in 7 days , but with the help of his son , he can do it in 3 days . in what time can the son do it alone ? 
Options: a ) 5 , b ) 7 , c ) 15 / 2 , d ) 21 / 4 , e ) 18 / 5
Output (English): d