Minute Mystery: The Two Hourglasses

Bite-sized brain teaser to solve in a minute or less.

 

 

The Two Hourglasses

Category: Logic, Time Puzzle

You have a 7‑minute hourglass and an 11‑minute hourglass. How can you measure exactly 15 minutes with no other timer?

 

 

Solution

  1. Start both hourglasses.
  2. When the 7‑minute hourglass runs out, turn it over immediately (7 minutes elapsed).
  3. When the 11‑minute hourglass runs out, turn over the 7‑minute hourglass again. At that moment, 11 minutes have elapsed in total and the 7‑minute hourglass has been running for 4  minutes since its last flip and so has 4 minutes to run after the second flip.
  4. When the 7‑minute hourglass runs out again, another 4 minutes would have elapsed, bringing the total time to 15 minutes.

 

 

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