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| Classic Brain Teasers and Riddles |
(Click on the question to see the answer.) |
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| 61. |
It takes 5 builders to build 5 houses in 5 days.
 If the ratios remain the same, how many days should it take 100 builders to build 100 houses? |
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ANSWER
It should still take 5 days for 100 builders to build 100 houses if all the ratios remain the same.
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| 62. |
Three students checked into a hotel and paid the clerk $30 for a room ($10 each). When the hotel manager returned, he noticed that the clerk had incorrectly charged $30 instead of $25 for the room. The manager told the clerk to return $5 to the students. The clerk, knowing that the students would not be able to divide $5 evenly, decided to keep $2 and to give them only $3. The students were very happy because they paid only $27 for the room ($9 each). However, if they paid $27 and the clerk kept $2, that adds up to $29. What happened to the other Dollar? |
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ANSWER
This is an example of adding the wrong value. The students ended up paying $27 which came from $30 - $3 = $27 and $27 + $3 is still a total of $30. The $2 the clerk kept is the additional amount the students should have also gotten back which would reduce the $27 down to the actual price of $25 (i.e. $27 - $2 = $25), the amount the students were suppose to be charged. |
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| 63. |
The four points represent vertices of a square. How can you move exactly two points to create the vertices of a square of exactly twice the area?
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| 64. |
You're on a plane to New York City. You want to know if it's snowing and the weather app on your phone doesn't always get it right. So you call 3 of your friends who live in New York City and ask each one if it's snowing. Each friend has a 2/3 chance of telling you the truth and a 1/3 chance of lying to you for fun. All 3 friends tell you, "Yes. It is snowing here." If those probabilities are accurate, what is the probability that it is in fact snowing in New York City at the moment? |
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ANSWER
The chance that all 3 lied is (1/3)^3 = 1/27. So, there is only a 1/27 chance that they all lied about it snowing. So the chance that just one or more of them is telling the truth is 26/27. This is a binomial probability situation of lying versus truth.
A more challenging question would be to ask what are the chances it is actually snowing if 2 friends said it was snowing and one said it wasn't. |
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GO TO CLASSIC BRAIN TEASERS PAGE 6 |
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