Beach Day by Rodger Bliss
April 2024
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Time This

If you have two hourglasses — one four-minute timer and one seven-minute timer, how can you measure nine minutes?

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6 guesses to Time This


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  • Julie

    Use the timer on the microwave.

  • Roolstar

    7M is the 7 minutes hourglass
    4M is the 4 minutes hourglass
    Tilt means rotate

    1- Tilt them both
    2- When the 4 minutes are up(3 minutes are left in the 7M) tilt the 4M alone
    3- When the 3 minutes are up in the 7M tilt it again (one minute is left in the 4M)
    4- When the 1 minute is over in the 4M (6 minutes are left in the 7M) tilt them BOTH
    5- When the 4 minutes are up, you’ve got 2 minutes in the 7M!

    With 2 minutes left in 7M, all you have to do is let them pass then tilt 7M again and you’ve got your nine minutes!

    So in fact, you’d have to wait 12 minutes before starting to measure 9! 21 minutes in total. Can we do it in a shorter time?

  • dude

    The RiddleDude can do it in exactly nine minutes, albeit the same amount of steps. Before revealing our method, we’ll wait and see if anybody else can step up to the plate with the correct answer.
    Nicely played, Rool!

  • dude

    Actually, upon closer inspection, you technically have more than five steps in your answer. We can do it in nine minutes and five steps.
    Badabing!

  • Roolstar

    Well, I was trying to prepare the timers for measuring 9 minutes: In fact with 2 minutes on one side of the 7M hourglass, I can rest it on the side until needing it.

    But if I want to start measuring right away, I’ll only change step 4 from my answer before and step 5 won’t be needed:

    1- Tilt them both
    2- When the 4 minutes are up(3 minutes are left in the 7M) tilt the 4M alone (4 minutes have already passed)
    3- When the 3 minutes are up in the 7M tilt it again (one minute is left in the 4M) (7 Minutes passed so far)
    4- When the 1 minute is over in the 4M (6 minutes are left in the 7M) tilt 7M and the remaining minute will be added (9 Minutes passed so far)

    Is that what you were looking for? 9 minutes in 4 steps?

  • dude

    Nicely played, Rool!
    You nailed it!
    Here’s our answer:

    1. Turn over both hourglasses at once.
    2. When the four-minute hourglass runs out, turn it over.
    3. When the seven-minute hourglass runs out, turn it over.
    4. When the four-minute timer runs out this time (eight minutes have elapsed), the seven-minute hourglass has been running for one minute.
    5. Now turn over the seven minute timer back over (remember, it had been running for one minute, by turning it over again, you have a one minute timer). When the seven minute timer runs out, nine minutes have elapsed.

    You’re today’s winner!