Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.
Imagine a water-cask and three different sized water-taps. The smallest tap can fill the water-cask in 20 minutes. The middle tap can fill the cask in 12 minutes. And the largest tap can fill the water-cask in 5 minutes. How long does it take to fill the water-cask with the three taps together?