Mathias' Candles

Mathias is an experienced waiter. He knows he must light one candle per hour to create a warm atmosphere.

What is the number of hours during which exactly three candles will be burning at the same time?

Make sure you watch the video at least twice before jumping into solving mode! Pay attention to the words: they play an important part. This puzzle is moderately difficult; the students should be able to solve it alone or in small teams.