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?
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.