A full tank can be emptied by its outlet in 30 minutes. An inlet that alone would fill the empty tank in 45 minutes is left running by mistake while the tank drains. How long does the tank take to empty?

Take the tank as 90 units: the outlet removes 3 units a minute and the inlet puts back 2, so the level falls by 1 unit a minute and the tank takes 90 minutes to empty. 30 minutes ignores the inlet. 18 minutes adds the two rates (90 ÷ 5) when the inlet works against the outlet and must be subtracted. 75 minutes adds the two times, which is not how opposing rates combine.