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?
My solution is 16 minutes until you have a 9 minute egg:
Start 7 and 4 together.
When 4 runs out, turn it and let it run for the last three minutes of 7.
When 7 runs out, START THE EGG, but let 7 sit (do not turn it) for one minute until 4 runs out the second time. The egg will cook for ONE MINUTE while this happens.
Turn both timers when 4 runs out.
When 4 runs out the third time, turn it and leave 7 running. The egg will cook for SEVEN MINUTES while 7 runs out.
When the 7 timer runs out, let it stop. The egg has cooked for EIGHT MINUTES now and 4 has one minute left to go on it's fourth turn.
When the 4 timer runs out the fourth time, your egg is done.
4 minutes X 4 turns = 16 minutes is the earliest you can enjoy the egg.
Q.E.D.
ππ
Solution:
