A tank is by pipe A in 5 hours, pipe B in 10 hours and pipe C 30 hours. How much time will all the pipes together take to fill the tank?
Answer: D
Part filled by A in 1 hour = 1/5
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 =
1/5+1/10+1/30
=1/3
So all pipes will fill the tank in 3 hours.