Task: Question Answering
Input (English): Problem: a water tank is one - fifth full . pipe a can fill a tank in 15 minutes and pipe b can empty it in 6 minutes . if both the pipes are open , how many minutes will it take to empty or fill the tank completely ? 
Options: a ) 2 , b ) 4 , c ) 6 , d ) 8 , e ) 10
Output (English): a