An inlet pipe can fill a tank in 12 minutes. A drain can empty the tank in 18 minutes. If the tank is empty, and both the pipe and drain are open, how long will it take before the tank overflows?

Answer 1

Let x = number of minutes it takes to fill the tank.

x / 12 = portion filled in x minutes

x / 18 = portion drained in x minutes

x / 12 - x / 18 = 1

36(x / 12 - x / 18) = 36

3x - 2x = 36

x = 36 minutes

Answer 2

I did the problem slightly differently, drew a picture and explained it more in-depth.

It's how I would like it explained if I were seeing this for the first time.