Pipe A can fill a tank in 10 hours, pipe B in 20 hours, and pipe C in 60 hours. If all the pipes are open, in how many hours will the tank be filled?	Select:
A) 6 B) 7 C) 8 D) 9 E) 10
The part filled by A + B + C in 1 hour is 1/10 + 1/20 + 1/60 = 1/6   All the three pipes together will fill the tank in 6 hours.   The answer is 6.
<result>A</result>
Pipe A can fill a tank in 10 hours, pipe B in 20 hours, and pipe C in 60 hours. If all the pipes are open, in how many hours will the tank be filled?