Task: Question Answering
Input (English): Problem: a large tank can filled by a and b in 60 minutes and 40 minutes respectively . how many minutes will it take to fill the tanker from empty state if b is used for half the time and a and b fill it together for the other half ? 
Options: a ) 10 min , b ) 15 min , c ) 30 min , d ) 25 min , e ) 42 min
Output (English): c