Pipe A can fill a tank in 5 hours, pipe B in 10 hours and pipe C in 30 hours. If all the pipes are open, in how many hours will the tank be filled?

A. 2
B. 2.5
C. 3
D. 3.5

Explanation:

Part filled by A + B + C in 1 hour = 1/5 + 1/10 + 1/30 = 1/3

All the three pipes together will fill the tank in 3 hours.