Task: Question Answering
Input (English): Problem: x and y can do a piece of work in 20 days and 12 days respectively . x started the work alone and then after 4 days y joined him till the completion of work . how long did the work last ? 
Options: a ) 8 days , b ) 10 days , c ) 12 days , d ) 13 days , e ) 14 days
Output (English): b