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 5 hours . if both the taps are opened simultaneously , then after how much time will the cistern get filled ? 
Options: a ) 7.5 , b ) 20 , c ) 30 , d ) 32 , e ) 40
Output (English): b