# Math Riddles

##### Math Riddle #131 (hard)

Question: If you have an 11 minute and 13 minute hourglass, how can you accurately time 15 minutes?

##### Math Riddle #129 (medium)

Question: There are two ropes that both take exactly 1 hour to burn from end to end. You are unable to cut the rope. How can you burn the two ropes in a total of 45 minutes?

##### Math Riddle #118 (medium)

Question: A shrimping boat is at the docks with a lifeboat hanging off the side 5 meters above the water. If the tides rise at a rate of 1.5 meters per hour, how long would it take for the water to reach the lifeboat.

##### Math Riddle #99 (hard)

Question: How many people do you need to have the odds be in favor (at least 50% chance) of two people having the same birthday?

##### Math Riddle #98 (hard)

Question: Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a sports car; behind all of the others is bicycles. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 2, revealing a bicycle. He then says to you, "Do you want to pick door No. 3?" Is it to your advantage to switch your choice?