The Next Letter
Which is the next letter in the following sequence:
S S E N T T T T T T T T T T T T T T T T T T T T ?
SOLUTION
The answer is F. These are the first letters of the numbers Seventeen, Eighteen, Nineteen, Twenty, Twenty-one, and so on.
Category: Brain Teasers
A collection of Math, Chess, Detective, Lateral, Insight, Science, Practical, and Deduction puzzles, carefully curated by Puzzle Prime.
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Eight Sticks
You have 8 thin sticks; 4 of them have length 1 and the other 4 have length 2. Arrange the sticks so that they form exactly 3 identical squares, without any leftovers.
SOLUTION
The solution is shown below.
Find the Mistake
CAN YOU FIND THE
THE MISTAKE?
1 2 3 4 5 6 7 8 9
SOLUTION
The word “THE” appears twice in the question. Read carefully!
Trotsky’s Killer
August 20, 1940
Leon Trotsky, an enemy of Josef Stalin, had been in exile for 11 years. On this day, a Soviet assassin entered Trotsky’s house in Mexico City and hit him in the head with an ice axe. He died the next day.
Here is a group of suspects who were rounded up. Each wrong suspect has one different feature from the correct suspect. Who killed Trotsky?
SOLUTION
The killer of Leon Trotsky was Ramón Mercader.
Canaries in Space
NASA was considering sending canaries into space to study them under zero gravity. The project was scrapped when someone realized that in spite of having sufficient water supplies, they could die of dehydration within a few hours. Why?
SOLUTION
Unlike humans, birds need gravity in order to swallow. Thus, in space they wouldn’t be able to drink and will die of dehydration.
53 Bicycles and a Dead Man
There is a room with a table, 53 bicycles, and four men. One of the four men is dead. How did he die?
SOLUTION
The “bicycles” are the (famous) Bicycle playing cards. The four men were playing poker, but one of them was cheating with an extra card, so he got killed.
The Ark
How many species of animals did Moses take on the Ark with him?
SOLUTION
None. It was Noah’s ark, not Moses’.
Plinks, Plonks, and Plunks
If all plinks are plonks and some plunks are plinks, which of these statements must be true?
- All plinks are plunks.
- Some plonks are plunks.
- Some plinks are not plunks.
Remark: “Some” means more than 0.
SOLUTION
The first statement says that the set of plonks contains the set of plinks, and the second statement says that there is at least one plunk-plink. Therefore, that plunk-plink must also be a plonk, and the second statement is true.
The first and the third statements, however, do not need to be true. Indeed, it is possible that there is a plink that is not a plunk, or that all plinks are plunks.
Lost Without Moving
In 1972 there was a chess game between two famous players. One of them lost the game without moving any pieces. How?
SOLUTION
It was the game in 1972 between Bobby Fischer and Boris Spassky, when Bobby Fischer’s clock ran out of time. Fischer himself even did not appear for the game due to his protest against the cameras installed in the playing hall.
Envelopes with Numbers
You are given 2 sealed envelopes with numbers inside. You are told that one of the numbers is twice as much as the other one. You grab one of the envelopes and right before you open it, you make the following calculation:
“If this envelope contains X inside, then the other envelope contains either X/2 or 2X inside. Since the chance that the other envelope contains a larger number is exactly 50%, the expected money I will get after switching is X/4 + X = 1.25X > X. Therefore, I should switch!”
Clearly, this reasoning is wrong, since you can’t possibly deduce which envelope of the two contains a larger number. Where is the mistake?
SOLUTION
The trick is that conditionally on the fact that your envelope contains X, it is not true that the other envelope has 50% chance of containing either X/2 or 2X. The reason is that it is impossible that all amounts of dollars appear in the envelopes with the same probabilities (densities). Thus, for example, if it is very unlikely that an envelope contains more than 1000, and you open an envelope with 800 inside, you will not think that the other envelope has 50% chance of containing 1600.