A tank of capacity 7,200 litres stands empty. An inlet delivering 240 litres a minute and a drain removing 90 litres a minute are opened at the same moment and both are left running. How many litres will the inlet have delivered by the time the tank is full?

The net gain is 240 − 90 = 150 litres a minute, so filling takes 7,200 ÷ 150 = 48 minutes. In that time the inlet delivers $240\times 48=11,520$ litres. 7,200 litres is the tank's capacity, which would be the inlet's output only if nothing ran to waste. 4,320 litres is what the drain threw away $(90\times 48)$. 19,200 litres divides by the drain's 90 litres a minute instead of by the net 150, stretching the filling to a false 80 minutes.