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?
(In reply to
Solution: by G. Steve Arnold)
Just realized something...
16-9=7, so you can throw the 7 timer back in the drawer the moment you start the egg - it doesn't provide any info to you beyond that point.
Updated (simpler) solution:
Start both timers together.
Turn 4 timer immediately whenever it runs out.
Start the egg when the 7 timer runs out the first time.
The egg is done when the 4 timer runs out the fourth time (16 minutes).
Q.E.D.
re: Solution:
