Task: Question Answering
Input (English): Problem: a large tanker can be filled by two pipes a and b in 60 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 ) 30 , b ) 15 , c ) 20 , d ) 25 , e ) 50
Output (English): a