Can You Solve the Hourglass Logic Puzzle?

The hourglass brain teaser is a classic logic puzzle in that it requires little math skills—just some logical, practical thinking. This logic problem is featured in the book "Fundrum My Conundrum: A Book of Riddles" by Raymond Epstein and Benjamin Kovler. Give this problem a shot before scrolling down to read the solution. Good luck!

Here's The Logic Puzzle

You have two hourglasses—one hourglass measures 7 minutes, and the other hourglass measures 4 minutes. How can you time 9 minutes using nothing but these two hourglasses? Yes, this is possible! Scroll down to read the solution.

Here's the Answer

Start both hourglasses at the same time. When the four-minute hourglass runs out, turn it over to start it again (four minutes have elapsed at this point). When the seven-minute hourglass runs out, turn it over to start it again (at this point, seven minutes total have elapsed). When the four-minute hourglass runs out for the second time, eight minutes have elapsed. At this time, the seven-minute hourglass has been running for one minute. Turn the seven-minute hourglass over again and wait for the one-minute's worth of sand to run out. Once that hourglass runs out, nine minutes will have elapsed.

If you'd like to try more challenging puzzles, check out "The 125 Best Brain Teasers of All Time: A Mind-Blowing Challenge of Math, Logic, and Wordplay" by Marcel Danesi. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

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Written by Curiosity Staff March 15, 2017

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