## Saturday, March 10, 2018

### 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?

## Saturday, March 3, 2018

### 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.

## Saturday, February 24, 2018

### Coin Flipping Game

You have ten coins in a row, all facing tails up. You are to perform a sequence of moves on the coins where one move consists of flipping over any one coin from tails to heads, then flipping over the coin to its immediate right (whether the second coin is heads or tails does not matter, just flip it over). Can you prove that no matter what moves you select, there are a finite number of moves in the sequence? (In other words, prove that you will always reach a state where there are no more legal moves.) Click below for the answer.

## Saturday, February 17, 2018

### A Knight, a Knave, and a Spy

You find yourself on the Island of Knights, Knaves, and Spies, a logical kingdom whose inhabitants always lie (Knaves), always tell the truth (Knights), or who can do either (Spies). You encounter three of said inhabitants, call them Alice, Bob, and Carol. You are told by your guide (a trustworthy Knight) that in this group there are one of each type of inhabitant, a Knight, a Knave, and a Spy. You ask the following questions.

To Alice you ask, "Are you a Knight?"

To Bob you ask, "Are you a Spy?"

"No," replies Bob.

Finally, you ask Carol, "Are you a Knave?"

"No," she says.

Can you tell from these answers who is a Knight, who is a Knave, and who is a Spy? Click below for the answer.

## Saturday, February 10, 2018

A king decides he is going to fire one of his advisers. He tells him that he has written "You're fired" on one slip of paper, and "You can stay" on another, and that the adviser is to choose one at random. The king has secretly written "You're fired" on both notes, but unbeknownst to the king, the adviser has found this out! How can the adviser keep his job without telling the king that he knows both notes are the same?

## Saturday, February 3, 2018

### Water Glasses

You have five glasses arranged in a row. The first two are empty and the last three are filled with water. By moving only one glass, can you arrange them so full and empty glasses alternate? Click below for the answer.

## Saturday, January 27, 2018

### Knights or Knaves?

You find yourself on the Island of Knights and Knaves, a logical kingdom whose inhabitants either always lie (Knaves) or always tell the truth (Knights). You encounter three of said inhabitants, call them Alice, Bob, and Carol. You ask the following questions.

To Alice you ask, "Is Bob a Knight or a Knave?"