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[Logic Quiz] Flexible Thinking and Creativity Are Key! From Classics to Tough Challenges, All in One Go
Logic puzzles that call for flexible thinking and free imagination!
In this article, we introduce a wide range of problems—from classic ones you might see on TV or in magazines to challenging brainteasers with a twist.
Critical thinking and reading comprehension are certainly important, but surprisingly crucial is letting go of assumptions!
That “Isn’t this obviously what it means?” is often wrong to begin with.
Take this opportunity to clear your mind and tackle the problems without being bound by unconscious assumptions!
[Logic Quiz] Flexible Thinking and Creativity Are Key! A One-Stop Introduction from Classics to Brain-Twisters (1–10)
There are 200 fish. 99% are guppies. How many fish need to be removed to make 98% guppies?
See the answer
100 animals
There are 200 fish in total. Of those, 99% are guppies, which means there are 198 guppies and 2 non-guppies. If you want to make the guppies 98%, you have to reduce the number of guppies. Since there are 2 non-guppies, if you reduce the guppies from 198 by 100, you’ll have 98 guppies and 2 other fish, making it 98%. Reduce by 100 just to lower it by 1%? It might be surprising, but that’s the interesting part of this problem.
Of the coins placed on the table, only 10 are heads up, and all the rest are tails up. While blindfolded, how can you divide the coins into two groups so that both groups have the same number of heads-up coins?
See the answer
Divide the coins into 10 random coins and the remaining coins, and flip all 10 coins over.
It doesn’t matter how many of the initial 10 coins are heads up. If n of the 10 coins are heads, then the remaining heads are 10−n. When you flip the 10 coins, the number of heads becomes 10−n, so both groups end up with the same number of heads: 10−n.
What is the largest number that can be made using the three digits 1, 2, and 3?
See the answer
3 to the 21st power
If it’s a three-digit number, 321 is the largest, but if it’s 3 to the 21st power, it becomes the larger number 10460353203.
There are pieces of paper labeled with the numbers 1, 2, and 3 placed face down. You can ask the questioner only one question, and the answer will be either “Yes,” “No,” or “I don’t know.” However, there is a question that allows you to determine the number. What is that question?
See the answer
If you add 0 or 1 to the numbers written on the paper, will the total become greater than 2?
If it’s 1, it won’t be greater than 2, so “No”; if it’s 2, it could be 2 or 3, so “Unknown”; if it’s 3, it will be greater, so the answer would be “Yes.”
There are two incense sticks that each burn out in one hour. How can you use them to measure 45 minutes?
See the answer
Light three points simultaneously: one stick at one end, the other stick at both ends. When the first stick burns out, light the remaining end of the other stick.
An incense stick lit at both ends should burn out in 30 minutes. If you light one end first, by the time it has 30 minutes of burn time remaining, lighting the other end will make it burn twice as fast, so 30 ÷ 2 = 15 minutes, and everything will burn out in a total of 45 minutes.
“There were two sunny days.” “There was only one cloudy day.” “There were two rainy days.” “No two consecutive days had the same weather.” “The day before a rainy day was not sunny.” Determine the correct weather for Monday through Friday.
See the answer
Rain → Sunny → Cloudy → Rain → Sunny
Because the day before the rain is cloudy, we know that the sequence includes “cloudy → rain.” The remaining days are two sunny days and one rainy day, and from the condition that the same weather does not occur consecutively, the order “rain → cloudy → rain → sunny → sunny” is derived.
There are four boxes, and at least one of them contains treasure. Condition 1: If there is treasure in box A, then there is also treasure in box B. Condition 2: If there is no treasure in box C, then there is also none in box D. Condition 3: If there is none in box C, then there is also none in box B. If you can choose only one box, which box should you select to obtain the treasure?
See the answer
Box C
Let's focus on box C, which has the most information. If the treasure is not in C, then it is also not in B or D. However, if it’s not in B, then it’s not in A either. In other words, if it’s not in C, it’s in none of the boxes. Therefore, assuming the treasure is in exactly one box, the answer is C.
I fell into a 30-meter-deep pit. You can climb 3 meters every hour, but immediately slip down 2 meters. How many hours will it take to escape?
See the answer
28 hours
It climbs 3−2=1 meter every hour. However, at the 27th hour it will be at the 27-meter mark, so if it then climbs 3 meters, it can escape without sliding down 1 meter. Therefore, the correct answer is 27+1=28 hours, not 30 hours!
One of three brothers secretly ate some sweets. The eldest said, “The second brother is the culprit.” The second said, “The youngest is the culprit.” The youngest said, “I’m not the culprit.” Only the one who secretly ate the sweets is lying. Who is the culprit?
See the answer
second son
If the eldest son is the culprit, then the second son must be lying, and if the third son is the culprit, then the eldest son must also be lying. This breaks the assumption that only the person who secretly ate the food is lying. If the second son is the culprit, then he is the one lying while the eldest and third sons are telling the truth. Therefore, the correct answer is the second son.
A ballpoint pen and an eraser together cost 110 yen. The ballpoint pen is 100 yen more expensive than the eraser. How much does the eraser cost?
See the answer
5 yen
Given that the eraser and the ballpoint pen together cost 110 yen, we have “eraser + (eraser + 100 yen) = 110,” which leads to “eraser = 5 yen.” It’s a question where people often mistakenly answer 10 yen, but if the eraser were 10 yen, the pen would be 110 yen, totaling 120 yen, right?