Nine identical taps running independently at a constant rate fill a reservoir in 8 hours. How long would 6 such taps take to fill the same reservoir?

Explication

The reservoir needs 9 times 8, or 72 tap-hours, and 72 divided by 6 gives 12 hours. The value 5.33 hours comes from scaling time in the same direction as the tap count, even though fewer taps must take longer.