# Math Riddles

##### 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?

##### Missing Dollar Riddle (medium)

Question: Three guests check into a hotel room. The clerk says the bill is \$30, so each guest pays \$10. Later the clerk realizes the bill should only be \$25. To rectify this, he gives the bellhop \$5 to return to the guests. On the way to the room, the bellhop realizes that he cannot divide the money equally. As the guests didn't know the total of the revised bill, the bellhop decides to just give each guest \$1 and keep \$2 for himself. Each guest got \$1 back: so now each guest only paid \$9; bringing the total paid to \$27. The bellhop has \$2. And \$27 + \$2 = \$29 so, if the guests originally handed over \$30, what happened to the remaining \$1?

##### Math Riddle #75 (medium)

Question: There are 100 coins scattered in a dark room. 90 have heads facing up and 10 are facing tails up. You cannot tell which coins are which. How do you sort the coins into two piles that contain the same number of tails up coins?