Pipe A can fill a tank in 6 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 ? Pick one.
A) 2.5 hours
B) 3.3 hours
C) 3.5 hours
D) 3 hours
E) None of these
Explanation:   Part filled by A in 1 hour = 1/6   Part filled by B in 1 hour = 1/10   Part filled by C in 1 hour = 1/30   Part filled by (A+B+C) in 1 hour =   1/6+1/10+1/30=9/30   So all pipes will fill the tank in 3.3 hours.   Option 3.3 hours
<result>B</result>
Pipe A can fill a tank in 6 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 ?