1. You have 25 horses, and you want to pick the fastest 3 horses out of those 25. In each race, only 5 horses can run at the same time because there are only 5 tracks. What is the minimum number of races required to find the 3 fastest horses without using a stopwatch?
2. You are given 8 pennies, 7 of which weigh exactly the same, but one penny weighs less than the other 7. You also have a judge scale. Find the one penny that weighs the least in less than 3 steps.
3. You have a birthday cake and have exactly 3 cuts to cut it into 8 equal pieces. How do you do it?
4. Three switches Three bulbs?
5. How would you weigh an elephant without using a scale?
6. Suppose you have a 3 liter jug and a 5 liter jug (this could also be in gallons). The jugs have no measurement lines on them either. How could you measure exactly 4 liter using only those jugs and as much extra water as you need?
7. Four people need to cross a rickety rope bridge to get back to their camp at night. Unfortunately, they only have one flashlight and it only has enough light left for seventeen minutes. The bridge is too dangerous to cross without a flashlight, and it’s only strong enough to support two people at any given time. Each of the campers walks at a different speed. One can cross the bridge in 1 minute, another in 2 minutes, the third in 5 minutes, and the slowest camper takes 10 minutes to cross. How can the campers make it across in exactly 17 minutes?
8. Given the previous puzzle, with 8 marbles, how would you generalize the solution to find the minimum number of weighings if you are given n marbles?
9. Using only those 2 fuses and the lighter, how would you measure a period of exactly 45 minutes?
10. Suppose 2 boys are walking in the woods and they decide to take a shortcut through a railroad tunnel. They had walked 2/3 of the way through the tunnel, but then something horrible happened: a train was coming in the opposite direction towards the 2 boys, and it was coming close to the other entrance of the tunnel. Each boy ran in a different direction to get out of the tunnel and avoid the incoming train. Each boy ran at the same exact speed of 10 miles per hour, and each boy managed to escape the train at the exact instant in which the train would have hit and killed him. If the train was moving at a constant speed and each boy was capable of instantaneous acceleration, then how fast was the train going?
- 30 miles/hour
11. Prisoner Hat Riddle
12. 3 Ants Triangle Puzzle
13. How would you find the number of squares on a chessboard?