You are a cook in a remote area with no clocks or other way of keeping time other than a four-minute and a seven-minute hourglass. On the stove is a pot of boiling water. Jill asks you to cook a nine-minute egg in exactly 9 minutes, and you know she is a perfectionist and can tell if you under cook or overcook the egg by even a few seconds. How can you cook the egg for exactly 9 minutes?
1. Flip both hourglasses over and drop the egg into the water.
2. When the 4-minute timer runs out, flip it over (4 minutes elapsed, 3 remaining on the 7-minute timer).
3. When the 7-minute timer runs out, flip it over. (7 minutes elapsed, 1 remaining in the 4-minute timer)
4. When the 4-minute timer runs out, flip the 7-minute timer over. (8 minutes elapsed. 6 minutes remained in the 7-minute timer, but flipping it over leaves one minute’s worth of sand on top. When it runs out exactly nine minutes will have elapsed.)
One evening a man is walking home from work and an insect flies into his ear. He tries to pry it out but it doesn’t work. Shaking his head isn’t successful either. Finally he managed to get it out. How did he do it?
Walking down the street one day, I met a woman strolling with her daughter. “What a lovely child,” I remarked. “In fact, I have a younger child as well,” she replied.
What is the probability that both of her children are girls?
1/2 probability. This has been know to cause raging debates and is known as one of the variations of the Boy or Girl paradox. This variation is more straightforward because knowing the position of the child leaves only two possibilities – the other child is a boy or a girl, each of which have a 1/2 probability.
Yes, there’s a 4th of July in every country. Of course they don’t treat it as a holiday as citizens of the United States do, but that’s not what was asked.
