Task: Question Answering
Input (English): Problem: a can finish a work in 21 days and b can do the same work in 15 days . b worked for 10 days and left the job . in how many days , a alone can finish the remaining work ? 
Options: a ) 6 , b ) 5 , c ) 5.5 , d ) 7 , e ) 8
Output (English): d