Task: Question Answering
Input (English): Problem: a tank can be filled by pipe a in 5 hours and by pipe b in 8 hours , each pump working on its own . when the tank is full and a drainage hole is open , the water is drained in 20 hours . if initially the tank was empty and someone started the two pumps together but left the drainage hole open , how long does it take for the tank to be filled ? 
Options: a ) 2.6 hours , b ) 3.6 hours , c ) 4.6 hours , d ) 5.6 hours , e ) none of these
Output (English): b