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?

Simple solution first.
"How quickly..can you boil an egg?"
Exactly nine minutes. Not that you will know that nine minutes have passed, but it will be boiled in exactly nine minutes. (and then probably get over-boiled since you weren't watching the time, but that wasn't a part of the question)

Second solution.
Seriously now, leaving the previous technicality aside, the solution is 16 minutes. Proceed as follows:

A = 7-minute hourglass
B = 4-minute hourglass

Minute 0: Set A and B.
Minute 4: B stops. Reset B.
Minute 7: A stops. Begin boiling the egg.
Minute 8: B stops. Reset B.
Minute 12: B stops. Reset B.
Minute 16: B stops. Stop boiling egg.

Explanation:
7+9=16
16 is a multiple of 4 (if this wasn't the case, you would need to find some number where 7x+2 is a multiple of 4).

Posted by Your Mom on 2002-07-09 07:52:22