A water tank can be filled by an inlet pipe in 9 hours. It takes 2 times as long for the outlet pipe to empty the tank. How long will it take to fill the tank if both pipes are open?

Answer 1

in 18 hours, the inlet pipe will allow in 2 times the capacity of the tank, while the outlet pipe will empty the 1 tank's capacity in 9x2 = 18 hours.

two tanks full came in, and one tank full went out leaving the tank full after 18 hours 2-1 = 1 full tank in 18 hours.

IF both are open, the tank fills in 18 hours

in one hour

in - out = fraction of the tank filled in one hour

1/9 - 1/18 = 2/18 - 1/18 = 1/18 of the tank is full in 1 hour

x/1 = 1/(1/18) = 18 = x = hours needed to fill the tank