Wang is a chef in a special restaurant where clocks are outlawed (not permitted).
But Wang has a 4-minute hourglass, a 7-minute hourglass, and a pot of boiling water.
A customer named Diao orders a 9-minute egg;
Diao is extremely picky and will not like it if you overcook or undercook the egg, even by a few seconds.

1. What is the least amount of time it will take Wang to prepare the 9-minute egg?
2. And how will Wang do it?

答案请用积分帖子给出，否则无效！
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参考答案：两个沙漏同时启动，4分钟后把4分漏倒过来，再过3分钟把7分漏倒过来，此时用时7分钟，4分漏剩1分钟沙，等4分漏再次漏完，用时8分，此时立刻把7分漏再次倒转，漏完即为9分钟。

[ 本帖最后由 alloncrazy 于 2008-4-23 23:36 编辑 ]