Task: Question Answering
Input (English): Problem: a and b can together finish a work in 40 days . they worked together for 10 days and then b left . after another 21 days , a finished the remaining work . in how many days a alone can finish the job ? 
Options: a ) 10 , b ) 25 , c ) 60 , d ) 30 , e ) 28
Output (English): e