Counterfeit Coins A counterfeit coin weighs 11 g. A genuine coin weighs 10 g. A stack of coins contains 10 coins. There are 9 stacks containing only genuine coins. There is 1 counterfeit stack containing only counterfeit coins. What is the fewest number of weighings?
Lightbulb Problem There are three lightbulbs in one room. There are three light switches in another room. The rooms are on different floors of a building. Can you determine how the switches and bulbs are connected using one trip?
Why is a manhole cover round? It can’t fall in. It rolls. It won’t damage cars. It uses the least materials.
Night Crossing Person 1 takes 1 minute. Person 2 takes 2 minutes. Person 3 takes 5 minutes. Person 4 takes 10 minutes. There is one flashlight. At most two people can cross at once. How fast can the four people cross?
Ants There are three ants at the three corners of a regular triangle. Each ant starts moving in a straight line toward another, randomly chosen corner. What is the probability that none of the ants collide?
Billiard Balls You have eight billiard balls. One of them is defective, meaning that it weighs more than the others. How do you tell, using a balance, which ball is defective in two weighings?
Library How would you locate a specific book in a big library? There’s no catalog and no librarian to help you.
Money Sharing Christina and Julia have $21 between them Christina has $20 more than Julia. How much does each person have?