Logic Riddles

 
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Troll bridge (medium)

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Question: You're going to a friends house and want to give them some brownies. On the way to their house you cross 5 bridges. At each bridge you must pay a toll of half of your brownies to the owner. But they are all very nice and give you back a brownie from what you give them.

How many brownies must you start with to give your friend 2 brownies?

Answer: Just 2.

Each time you give the owner of a bridge half of your brownies (1 brownie) they give you one back as well.

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Question: There are 1600 people sitting around a circular table. The first person (person 1) has a sword and kills the second person then hands it to the next alive person (in this case person 3). Person 3 stabs person 4 and gives the sword to person 5. This goes on until person 1499 kills person 1500. Then person 1 kills person 3 and so on. This is repeated until there is only a single person remaining.

Who remains in the end?

Answer: Person 1153.

If you have any number of people equal to a power of 2 (2, 4, 8, etc.) then the first person will be the last remaining. The closest power of 2 to 1600 is 1024 (210). So the first person to go of the 1600 when there is 1024 people left will be the last person remaining. 1600 - 1024 = 576. 576 * 2 = 1152. Person 1152 will be the 576th person killed and person 1153 will be the first person to go of the remaining 1024 people.

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Question: You are in a car with some helium balloons and hit the gas. While you accelerate, which way does the balloon move?

Answer: It moves forward towards the front of the car, even though you are forced backwards. The helium balloon is less dense than the air so when you accelerate the air in the car will move backwards compared to the balloon pushing the balloon forward because the balloon is lighter.

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3 coins (medium)

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Question: You have three coins. One always comes up heads, one always comes up tails, and one is just a regular coin (has equal change of heads or tails). If you pick one of the coins randomly and flip it twice and get heads twice, what is the chance of flipping heads again?

Answer: 90 percent. If you pick the heads coin the chance of getting the first 2 heads is 100 percent (4/4) and if you pick the fair coin the chance is 25 percent (1/4). So from this, the chance that it is the heads coin is 4/5 and the fair coin 1/5.

Then, if it is the heads coin you will definitely get heads (4/5). If you flip the fair coin it is 1/10 (1/5 * 1/2). Add the probabilities together: 4/5 + 1/10 = 9/10.

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Question: You have an 8 by 8 chess board that has two corner squares diagonal from each other removed. Is there any way to place 31 dominoes of size 2x1 to cover the remaining 62 spaces?

Answer: No. When you remove the two diagonal spaces from the chess board they are both black or both white, making it so that there are two more of the other color. Each domino would cover 1 black and 1 white space. So 31 of them would have to cover 31 white spaces and 31 black spaces but with the corners removed there are 30 of one color and 32 of the other.

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