You have to boil a single egg for exactly nine minutes.

All you have are two hourglasses, able to measure (in sand) seven and four minutes, respectively.

How quickly, using only these two hourglasses, can you boil an egg?

Turn over both of the hour-glasses. When the 4-minutes hourglass is emptied, Turn it over again( 4 minutes passed).After that, when the 7-minutes hourglass is emptied, turn it over(3 minutes passed). When the 4-minutes hourglass is emptied again, turn over the 7-minutes hourglass(that's where you get the 1 minute from!).
Total time=4+4+1=9

Posted by christopher hii on 2002-07-12 03:18:52