[Slick]

Originally posted by Jack Simth:

quote:

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Originally posted by Slick:
Ok, here's neat little puzzle...

You have 2 egg timer sand-type "hour" glasses. One measures exactly 4 minutes, the other measures exactly 7 minutes. You are making a recipe requiring exactly 9 minutes. How do you time it.

Answers to all the weasel questions:
no you can't estimate times
no you can't lay one down on its side
no fancy stuff, this is not that hard.

Slick.

Can't put one sideways to pause it? Pity. No matter....

1) Start both timers.
2) When the four-minute timer finishes (three left in 7), flip 4.
3) When 7 finishes (three minutes later; one left in 4), flip 7.
4) When 4 finishes (one minute later; six left in 7) flip 4.
5) When 4 finishes (four minutes later; two left in 7) start recepie
6) When 7 finishes (2 minutes later), flip 7
7) When 7 finishes, (7 minutes later; 9 since started recepie), stop recepie.

This may be slightly more complex than is necessary, but should get the job done.

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Yes, this is a correct answer. Good job. There is another, shorter method that does not require setup: If you change step 4) to read: "When 4 finishes (one minute later; six left in 7) flip 7.", it works. (8 minutes have gone by at this point, and there is one minute of sand in the bottom of the 7 minute timer, so you can dump it back out to get to time the Last minute for a total of 9 minutes).

Slick.