Math Riddles

 
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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.

Answer: The entire shrimping boat would rise with the tides so the lifeboat would never be reached.

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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?

Answer: Start one of the ropes on fire from both ends and the other one from one end. After a half an hour one rope will be completely burnt and the other will be half burnt. At this point you light the final rope from the unlit side and it will take 15 minutes for the remaining half of the rope to burn, for a total of 45 minutes.

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

Answer: Start both hourglasses then when the 11 minute hourglass has finished immediately flip it again. When the 13 minute hourglass runs out the 11 minute hourglass will have 9 minutes left, so flip it and it will last another 2 minutes, 13 minutes + 2 minutes = 15 minutes.

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Question: On a clock, how many times a day do the minute and hour hand overlap?

Answer: 22 times: 12:00:00, 1:05:27, 2:10:55, 3:16:22, 4:21:49, 5:27:16, 6:32:44, 7:38:11, 8:43:38, 9:49:05, 10:54:33. Each twice a day.

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Question: You have a glass of water that looks about half full. How can you tell, only using the glass of water itself, if the glass is half full or not?

The glass is a right cylinder.

Answer: Tip the glass of water until the water reaches the rim of the glass and if the water lines up perfectly with the bottom rim of the glass, it is half full.

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