Three pipes, P, Q, and R can fill a tank in 10, 15, and 30 hours respectively. All three pipes are opened together for 1 hour, then pipe R is closed. How much time will it take for pipes P and Q to completely fill the tank?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 4 hours.
Let the capacity of the tank = LCM of 10, 15, and 30 = 30 unit
efficiency: P = 30/10 = 3 unit/h,
Q = 30/15 = 2 unit/h
R = 30/30 = 1 unit/h
in 1 hour = 3 + 2 + 1 = 6 unit
Remaining pipe fill by P and Q,
time = (30 - 6)/5 = 24/5 = 4 hours