Question

A tap can fill a tank in 15 minutes.Another tap can empty it in 20 minutes.Initially the tank is empty.If both the taps start functioning,when will the tank become full?

Answer 1

Answer: (A) 1 hour 

Solution:-
A tap can fill a tank = 15 min
Therefore, A tap can fill a tank in a min = 1/15

Another tap can  empty same tank = 20 min
Therefore, Another tap can empty a same tank = 1/20

If both the taps start functioning at the same time, 
Then, time taken by both tap to fill tank in a min = 1/15 – 1/20
 = (4 – 3)/60
 = 1/60

Both tank fill the tank = 60 min.