Task: Question Answering
Input (English): Problem: a can do a work in 20 days and b in 30 days . if they work on it together for 4 days , then the fraction of the work that is left is : 
Options: a ) 1 / 3 , b ) 2 / 3 , c ) 4 / 3 , d ) 5 / 3 , e ) 7 / 3
Output (English): b