Task: Question Answering
Input (English): Problem: it takes avery 4 hours to build a brick wall while tom can do it in 2 hours . if the two start working together and after an hour avery leaves , how much time will it take tom to complete the wall on his own ? 
Options: a ) 20 , b ) 40 , c ) 30 , d ) 50 , e ) 60
Output (English): c