Filling capacity = 1/time taken to completely fill a tank
Emptying capacity =1/time taken to completely empty a tank
Filling -Emptying Capacity=Portion of the tank filled in an hour
Emptying-filing capacity =Portion of the tank emptied in an hour
Let there be x filling pipes and y drain pipes.
(1/12)*x -(1/24)*y =1/2.....(ii)
(i) and (ii) we get
=>x= 8 and hence y-12-8=4
there are 4 drain pipes.