Pipe A can fill a tank in 4 hours, pipe B in 12 hours and pipe C in 40 hours. If all the pipes are open, in how many hours will the tank be filled ? Choose the most appropriate option:
A) 2.8 hours
B) 2 hours
C) 3.5 hours
D) 3 hours
E) None of these
Explanation:   Part filled by A in 1 hour = 1/4   Part filled by B in 1 hour = 1/12   Part filled by C in 1 hour = 1/40   Part filled by (A+B+C) in 1 hour =   1/4+1/12+1/40=43/120   So all pipes will fill the tank in 2.8 hours.   Option 2.8 hours
<result>A</result>
Pipe A can fill a tank in 4 hours, pipe B in 12 hours and pipe C in 40 hours. If all the pipes are open, in how many hours will the tank be filled ?