The Classic Hourglass Puzzle

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Here is a classic puzzle. Say that you have two hourglasses of different sizes. The bigger hourglass runs for 11 minutes while the smaller one runs for 7 minutes. Using these two hourglasses, how can you measure 15 minutes exactly.?

Solution

Step 1: Start the two hourglasses at the same time.

Step 2: When the 7-minute hourglass finishes, quickly turn it over without wasting a millisecond. At this point, 7 minutes had gone by.

Step 3: When the 11-minute hourglass runs out, the bottom part of the 7-minute hourglass has 4 minutes worth of sand. Now, quickly turn over the 4-minute hourglass. At this point, 11 minutes had gone by.

Step 4: Just wait for the turned over 7-minute hourglass to finish its 4 minutes worth of sand. Once it runs out, exactly 15 minutes will have gone by assuming that you turned over the hourglasses at a nigh-superhuman speed.

This is the only optimal solution for solving this puzzle. Well, you could always buy a clock or a watch I suppose.

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My name Edmark M. Law. I work as a freelance writer, mainly writing about science and mathematics. I am an ardent hobbyist. I like to read, solve puzzles, play chess, make origami and play basketball. In addition, I dabble in magic, particularly card magic and other sleight-of-hand type magic. I live in Hong Kong. You can find me on Twitter` and Facebook. My email is edmarklaw@learnfunfacts.com

18 thoughts on “The Classic Hourglass Puzzle

  1. flip both hourglasses at the same time and start dropping sand. when all the sand of the smaller hourglass has fallen out, begin to measure the time. and then the remaining sand of the bigger one has fallen out, flip it immediately. and when all of the sand in the bigger hourglass has fallen out, stop measuring the time… is this correct?

    Liked by 2 people

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