Task: Question Answering
Input (English): Problem: a cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 6 hours . if both the taps are opened simultaneously , then after how much time will the cistern get filled ? 
Options: a ) 17.5 , b ) 7.4 , c ) 7.9 , d ) 12.0 , e ) 7.1
Output (English): d