Pipe P can fill a tank in 4 hours, pipe Q in 8 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled?
Answer: B
Part filled by (P + Q + R) in 1 hour = (1/4 + 1/8 + 1/24) = 10/24
All the three pipes together will fill the tank = 24/10 = 2.4 hours