Pipe A can fill a tank in 10 hours, pipe B in 8 hours, and pipe C in 40 hours. If all the pipes are open, in how many hours will the tank be filled?  Choose the correct choice from the following answers:
A) 2
B) 3
C) 4
D) 5
E) 6
The part filled by A + B + C in 1 hour   = 1/10 + 1/8 + 1/40   = 10/40 = 1/4   All the three pipes together will fill the tank in 4 hours.   The answer is 4.
<result>C</result>
Pipe A can fill a tank in 10 hours, pipe B in 8 hours, and pipe C in 40 hours. If all the pipes are open, in how many hours will the tank be filled?