There is a water-cask with three different water-taps. With the smallest tap, the water-cask can be filled in 20 minutes. With the middle tap, the water-cask can be filled in 12 minutes. With the largest tap, the water-cask can be filled in 5 minutes.

The question: How long does it take to fill the water-cask with the three taps together?

The answer: Click here!