Friday, September 02, 2005

Do you know what the time is?

You have two hourglasses: a 7 minute one and an 11 minute one. Using just these hourglasses, accurately time 15 minutes.


  1. Start them at the same time. When the 7 minute one runs out, you've got 4 minutes in the 11 minute timer. Flip them both. When the 4 minutes runs out, you've got 3 minutes in the 7. Let the 11 minute run out. Flip them both again. When the 3 minutes runs out, stop the 11. You now have the 7 minute timer and 8 minutes of sand in the 11 minute one for a grand total of 15 minutes

    Or, you could run to Wal-Mart and grab a cheap digital watch for $5 and avoid word problems altogether. :)

  2. I hate Wal-Mart! I'd much rather head out to Target! ;-)

    Flip over both the seven minute hourglass (S) and the eleven minute glass (E) at the same time. When S is done, there are 4 minutes left in E. Flip over S immediately. When E is done there is 3 minutes left in S. Flip over S for another 4 minutes. That's 7+4+4 = 15.

  3. Start the 7 minute and the 11 minute at same time. When the 7 minute one runs out, you've got 4 minutes in the 11 minute timer.
    Forget about 7 minute timer.
    Flip the 11 minute timer to consume 4 minutes and then filp it again to measure 11 minutes.
    Therefore 4 + 11 = 15

  4. Wow! Someone came in with another answer. Thanks for that, Gaurav!


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