math, psychology, puzzle

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you have a 5 minute hourglass and a 9 minute hour glass. How can you use them to time a 13 minute barbeque?

  • math, psychology, puzzle -

    Start both at the same time. When the 5 min is done, you have 4 min left on the larger glass, start the bbq. When that hourglass is done, flip it over for the next nine min.

    4+9=13

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