A $180 \mathrm{~cm}$ high, horizontally based, cylindrical fuel tank is equipped with a tap at the bottom and a filling nozzle at the top. The latter can fill the tank in 1 hour. If we open both the filling nozzle and the tap for 5 minutes, the fuel level in the tank rises by $10 \mathrm{~cm}$. How long will it take to empty a full tank through the tap?
If both pipes are open, the level rises by $10 \mathrm{~cm}$ in 5 minutes, so in 1 hour with both pipes open, the gasoline level rises by 120 cm. Since the filling pipe raises the level by $180 \mathrm{~cm}$ in 1 hour, the outflow pipe must lower it by $180-120=60 \mathrm{~cm}$ in 1 hour. Therefore, the entire $180 \mathrm{~cm}$ column of gasoline can be drained in 3 hours.

Körmendy Attila (Bp. VIII., Kandó híradásip. t. I. o . t.)

Note: We can solve the problem with the same line of reasoning if we infer other (e.g., 1-minute or 5-minute) performance rates instead of the 1-hour rate.