9 Minutes

Hourglass

Question: You are given two hourglasses. One measures 4 minutes and one measures 7 minutes. How would you measure exactly 9 minutes?

Answer: This question is similar to 4 Quarts of Water. With water, we were able to fill and pour out the pail multiple times. We can use the same logic and easily come up solutions which start measuring 9 minutes somewhere in the middle of the procedure but those will not be ideal. If possible, we want to measure the 9 minutes right from the start.

Step Time 4 minute timer 7 minute timer
1 0 min Start Start
2 4 mins Flip 3 minutes left
3 7 mins 1 minute left Flip
4 8 mins Stop Flip (1 minute left)
5 9 mins Stop

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3 Comments

3 Responses

  1. I have better solution:
    1. Start 4, Start 7
    2. Flip, 3 mins
    3. 1min, Stop
    4. flip
    5. flip

    (1 + 4 + 4 )

  2. Howard says:

    I have been a reader for a long time, but am a first time commenter. I just wanted to say great post!

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