CONCEPT: Pipes and Cisterns

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

SOLUTION: Let there be x filling pipes and y drain pipes.

Then x +y=12=>y=(12-x)....(i)

Also,

(1/12)*x -(1/24)*y =1/2.....(ii)

Solving (i) and (ii) we get

x/12 -(12-x)/24=1/2

=>x= 8 and hence y-12-8=4

Thus there are 4 drain pipes. (A)