BrainDen » Puzzles

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BrainDen is a place to discuss new brain teasers as well as old brain teasers. This section of the forum offers space to discuss puzzles that are logical or mathematical.

BrainDen » Puzzles

2w ago

There are two bowls that you and a challenger must eat from. After flipping a coin you were selected to pick the bowl that each would eat from. In the first bowl there are three out of five poisonous apples. In the second bowl, there are two out of five poisonous apples. Whoever eats from the first bowl must eat two apples at random from the bowl. Whoever eats from the second bowl must eat three random apples from the second bowl. Which bowl should you pick to eat ..read more

BrainDen » Puzzles

1M ago

On the square A1 of a regular chessboard is a regular dice, face 1 up and face 2 in front. The only allowed move is to rotate the dice by 90 degrees to an adjacent square.
Find the shortest way from A1 to B1 so that the dice ends in the same orientation (face 1 up and face 2 in front).
Inspired by https://www.chiark.greenend.org.uk/~sgtatham/puzzles/js/cube.html ..read more

BrainDen » Puzzles

4M ago

A robot is placed on the square A1 of a standard chessboard and has to reach H8. It understands orders Up, Down, Left, Right. On some squares, there might be a cement block; is such a case, the robot does not execute the order and continues with the next one on your list.
There always is at least one possible path.
The list of directions is finite.
The robot might reach the destination somewhere in the middle of your list.
Give the answer in the form UURRU.
(This list will work i.e. on an empty 3x3 chessboard or with a single block on A3 but will fail with a single block on B3 ..read more

BrainDen » Puzzles

5M ago

Take from a pack of cards all the Aces, Kings, Queens and Jacks.
Arrange them in a 4 × 4 square so that every row, column and diagonal contains one card of each value (A,J,Q,K) and one card of each suit (Heart, Spade, Diamond, Club ..read more

BrainDen » Puzzles

7M ago

Hello everyone,
I’ve created a little puzzle that follows the cryptographic principle of zero-knowledge proof.
Let P = xx, the age of Peter
To find xx, I will provide you with means to verify the statements of the puzzle, without giving you any direct informations about the ages of the characters.
The ages of the characters are not given but can be found.
Although there are an infinite number of answers that could verify the information I provide, there is one answer that can be verified to 99% assuming the puzzle is honest and verifiable, and that Peter has a realistic age and life.
&n ..read more

BrainDen » Puzzles

8M ago

In the Garden of Eden, there is a circular pathway, where the Angel and the Devil enjoy an infinite stroll, walking side by side. There are lanterns placed along the pathway (a finite number of them). Each lantern can be in one of two states: on or off. The Angel, a servant of light, has control over the illuminated lanterns. Whenever they pass such a lantern, the Angel decides whether it remains on or off. Conversely, the Devil, a servant of darkness, has control over the lanterns that are dark, and can change their states as they pass.
To alleviate their boredom, the Angel and the Devil dec ..read more

BrainDen » Puzzles

10M ago

Every morning, I brew 3 cups of coffee in my French press. I prepare a large mason jar with ice, sweetener, and cream which fills the container half-way. I add enough coffee to fill to the top of the jar. Throughout the morning I drink the jar down halfway just to top it up once again. I am able to refill it twice fully. For the third refill, I am only able to add 2 FL oz. of coffee. How big is my Mason Jar?
Bonus question: what % of coffee is in my last cups mixture after adding the 2 oz?
For added clarity, when I top it up, I mean that I am only adding my ..read more

BrainDen » Puzzles

1y ago

Suppose we have a quadrilateral with Angles A,B,C,D, corresponding sides a,b,c,d, and the following fact:
CosA/a=CosB/b=CosC/c=CosD/d
What can be claimed about this quadrilateral ..read more

BrainDen » Puzzles

1y ago

Assume that n is a natural number, prove that n and n5 will always have the same one's digit. e.g. 13 and 135=371,293 both end in 3 ..read more