Task: Question Answering
Input (English): Problem: a man can do a piece of work in 10 days , but with the help of his son , he can do it in 4 days . in what time can the son do it alone ? 
Options: a ) 13 / 2 , b ) 17 / 3 , c ) 15 / 2 , d ) 20 / 3 , e ) 9
Output (English): d