Task: Question Answering
Input (English): Problem: x can do a piece of work in 8 hours . y and z together can do it in 6 hours , while x and z together can do it in 4 hours . how long will y alone take to do it ? 
Options: a ) 24 hours , b ) 20 hours , c ) 34 hours , d ) 12 hours , e ) 10 hours
Output (English): a