Question

a tank is filled with pipe A in 20 minutes. and pipe B in 30 minutes. When filled,it can be vacated by pipe C in 10 minutes. If all three pipes are opened simultaneously. ⅔rd of tank can be filled in how much time?

Answer 1

Pipe A takes 20min to fill; in 1 min it will fill 1/20 part.

Pipe B takes 30 min to fill; in 1 min it will fill 1/30 part.

Pipe C takes 10 min to vacate; in 1 min it will vacate 1/10 part.

Combinedly in 1 min it will result in:

1/20 + 1/30 - 1/10

or, (3 + 2 + 6)/60

or, 11/60 part of the tank.

The complete tank can be filled in 60/11 mins = 5.45 minutes.

2/3 part can be filled in (2/3 * 60/11) = 3.63 minutes.