Filling a Tank

[DEvans]

Hi, would you please be so kind as to help me with this word problem?

Filling a tank. A water tank has an inlet pipe and a drain pipe. A full tank can be emptied in 30 minutes if the drain is opened and an empty tank can be filled in 45 minutes with the inlet pipe opened. If both pipes are accidentally opened when the tank is full, then how long will it take to empty the tank?

t = full tank
30 minutes
45 minutes
e = empty tank

Thank you for your help.
D

 

[Gene]

In one minute you will drain 1/30 of the tank but fill 1/45.
-1/30+1/45 =
-3/90+2/90 = -1/90 per minute