Task: Question Answering
Input (English): Problem: a and b can finish a work 30 days if they work together . they worked together for 20 days and then b left . a finished the remaining work in another 20 days . in how many days a alone can finish the work ? 
Options: a ) 30 , b ) 60 , c ) 40 , d ) 20 , e ) 50
Output (English): b