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Repetitive

You have two groups of words:

  1. black, word, English, brief, noun, grandiloquent, last
  2. white, number, Russian, long, verb, unpretentious, first

To which group does “repetitive” belong?

The first group contains self-explanatory words (known as autologicals), the second group does not. Therefore “repetitive” should belong to the first group.

Chocolate Bar

Louis has a bar of chocolate 4×6 which is marked into 24 little squares. At each step, he breaks up one of its pieces along any of the marked horizontal/vertical lines. Show that no matter how he does that, it will always take the same number of steps until the chocolate is broken into single 1×1 pieces.

Every time he splits the chocolate, the number of pieces increases by 1. Therefore it will always take him 23 steps to split it into single pieces.

Twiddled Bolts

Two identical bolts are placed together so their grooves intermesh. If you move the bolts around each other as you would twiddle your thumbs, holding each bolt firmly by the head so it does not rotate and twiddling them in the direction shown below, will the heads:

(a) move inward
(b) move outward, or
(c) remain the same distance from each other?

One of the bolts will be screwing itself, and the other one will be unscrewing itself. This will happen at the same pace and the bolts will remain the same distance from each other. Thus the answer is (c).

The Missing Dollar

Three people check into a hotel room and each of them gets charged $10 – a total of $30. Later the clerk realizes that the bill is just $25, so he sends the bellboy to return $5 to the guests. On his way to the room, the bellboy decides to cheat and pockets $2 of the money, and gives the three men just one dollar each. Now the three men have spent $9 each, for a total of $27. Additionally, the bellboy took $2 for himself, which adds up to $27 + $2 = $29. Since the guests originally handed over $30, the question is what happened to the remaining $1?

The calculation is made the wrong way. The three men originally gave $30, but later $5 of them were sent back, which makes it $30 – 5 = $25 left at the clerk. Each of the men spent $9, so they gave $27 in total, $2 of which ended up in the bellboy’s pocket. $27 – $2 = $25, so no “missing dollar” here.

3 x $9 – $2 = $30 – $5.

Lab Mice

A scientist has 9 bottles, exactly one of which contains poison. The poison kills any creature which drinks it within 24 hours. If the scientist has 2 lab mice at his disposal, how can he find which is the poisonous bottle within 2 days only?

Label the bottles B1, B2, B3, … , B9.
The first day he lets the first mouse drink B1, B2, B3, and let the second mouse drink B1, B4, and B5. If after 24 hours both mice die, then the poisonous bottle is B1. If only one mouse dies, say the first one, then he lets the second mouse drink B2. If it dies, then the poisonous bottle is B2, otherwise, it is B3. Finally, if neither mouse dies, then he lets the first mouse drink B6 and B7, and lets the second mouse drink B6 and B8. If both mice die after 24 hours, then the poisonous bottle is B6. If only one mouse dies, say the first one, then the poisonous bottle is B7. If neither mouse dies, then the poisonous bottle is B9.

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Y-Town

In Y-town all crossroads are Y-shaped, and there are no dead-end roads. Is it true that if you start from any point in the city and start walking along the roads, turning alternatingly left and right at each crossroad, eventually you will arrive at the same spot?

Yes, it is true. If you start walking forward, eventually you will end up in a loop. It is easy to see that your entire path, including the starting spot, must belong to this loop. Therefore, eventually you will end up in the starting spot again.