Mismatched Letters

Soon after her wedding, a young bride hand-writes 100 thank you letters to all of her recent wedding guests and addresses 100 matching envelopes. Being in a hurry to get them in the mail, her new husband randomly stuffs the letters into envelopes and mails them out. What is the probability that exactly 99 of the letters made it into the right envelope?

This problem may seem difficult to model, but don't worry, it isn't necessary. If 99 out of 100 letters are in the right envelope, then the 100th one must be as well. So there is a probability of 0 that exactly 99 are correct.

Flipping 100 Coins

You have 100 fair coins and you flip them all at the same time. Any that come up tails you set aside. The ones that come up heads you flip again. How many rounds do you expect to play before only one coin remains? What if you start with 1000 coins? Click below for the answer.

When you flip 100 coins in the first round, you should expect to see about 50 heads. When you flip those 50 coins again in the second round, you should expect about 25 heads. This (approximate) halving pattern will continue until you're left with only one coin. On average, it will take 6 or 7 rounds when starting with 100 coins because $$log_2(100) = 6.644$$
When you start with 1000 coins, the game will last about 10 rounds. (So if you want to bet someone that you can flip a coin and have it come up heads 10 times in a row, you'll greatly improve your odds if you start with 1000 coins!)

See my Probability GitHub repository for a script that shows how to model this problem in Python.