Logic Riddles

 
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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: You work at a factory that produces dumbbell weights for lifting. One of the 10 machines has been producing weights that are always 1 pound less than the rest of the machines. You have a large scale used to weigh the weights for quality control. How could you find the defective machine by using the scale only once? You are allowed to make as many weights with the machines as you please.

Answer: From each machine you make a different number of weights and however many pounds the scale is off from what it should be corresponds to the machine that made that many weights.

From machine 1 you make 1 weight, machine 2 you make 2 weights...

If the machine is off by 1 pound you know it was machine 1 that was off.

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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: There are three boxes labeled "Apples", "Oranges", and "Apples and oranges". Each box has the wrong label on it. How could you correctly label all three of the boxes by taking only one piece of fruit fruit from one of the boxes?

Answer: A piece of fruit would be taken from the "Apples and oranges" box. Since this is the wrong label you know whatever fruit you draw from this box is all the same and you can give that box the proper label. Since all of the boxes started out wrong you know that the last box with its original label is wrong and can be labeled "Apples and oranges". From here you can just put the final label on the final box.

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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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