I have 2 hourglasses. One measures 7 minutes and the other measures 11 minutes. What is the quickest way to use them to time the boiling of an egg for 15 minutes?
If you haven’t tried to solve it, have a go now. For everyone else the answer is after the break.
Start the 2 hourglasses together. When the 7 minute hourglass finishes, turn it over. When the 11 minute hourglasses finishes, turn the 7 minute hourglass over again. When the 7 minute hourglass finishes the 15 minutes will be up.
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.