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Cooking Time

A cook must put a souffle exactly 9 minutes at the oven, but he only have a 7 minutes hourglass and a 4 minutes hourglass.

How can he measure 9 minutes with those clocks?

see explanation

Turn at the same time both hourglasses. When the 4 min hourglass finish, turn it again. When the 7 min. hourglass finish, turn it. At this time 8 min. have pass and only one minute is needed. When the 4 min. hourglass finish, turn again the 7 min. hourglass. The sand that have fall is going to last one min. and you have count 9 min. in total.



Turn both 4 min and 7 min hourglasses. When 4 min hourglass finishes ends you have: 4 minutes passed. 7 min hourglass has 3 minutes of sand. Turn the 4 min hourglass. When the 7 min hourglass finishes you have: 7 minutes passed. 4 min hourglass has 1 minute of sand. Turn the 7 min hourglass. When the 4 min hourglass finishes you have: 8 minutes passed. 7 min hourglass has 6 minutes of sand (which means you have 1 minute of sand on the other side). Turn the 7 min hourglass again. You have 1 minute of sand so that makes 9 minutes when it finishes.


WRONG! You said, "At this time 8 minutes have passed." No, only seven minutes have passed. It can't be done. Ten Minutes yes, but not 9.



Check my reply for a more detailed explanation


Let the 4 min one flow, then let the 7 one flow for 5 minutes

Todd Gibler

Before you start the souffle turn both at the same time when the 4min one is done turn it back over thus it it at 4 min let the seven and the 4 run when the 7 I out there will be 3 left in the 4 do the same thing again and the 4 min will have 2 min when it is there put it in let the 4 run out flip the 7 and done

Kvk Venugopal

Good explanation

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Submitted by
over 5 years ago
Difficulty 5.7 ?