Task: Question Answering
Input (English): Problem: a and b can together finish a work in 30 days . they worked together for 20 days and then b left . after another 20 days , a finished the remaining work . in how many days a alone can finish the job ? 
Options: a ) 40 , b ) 50 , c ) 54 , d ) 60 , e ) none of these
Output (English): d