Brain Teasers and Puzzles

Brain teasers, puzzles, riddles, and other challenges

Difficulty - Easy, Word Puzzles

Compound Word Puzzle 1

Compound words are words that are composed of two or more other words, like high + light = highlight.

Can you make compound words out of the following three words by adding the same 4-letter word to each of them?

WOOD
FALL
SURF

Difficulty - Easy, Simple Math

I am thinking of a 3-digit number. When you reverse the digits of this number, you get a bigger number, and if you add that to the original number, the sum is 463. What was the number I was thinking of?

Difficulty - Medium, Interview Brain Teasers

Card Flipping Endgame

There are n playing cards lined up face-down in a row. Every turn, a pair of adjacent cards with the left card face-down is randomly selected (i.e., a pair of cards has no chance of being selected if the left card is face-up, otherwise all pairs are equally likely to be selected). Both cards are then flipped over (face-down to face-up or face-up to face-down).

Prove that after enough turns, it will eventually be impossible to select a pair of cards with the left card face-down.

When you reach this card flipping endgame, will the rightmost card be face-up or face-down?

Difficulty - Easy, Physics and Science

Drink Mix Riddle

Two kids are playing around with their drinks at lunch. One has 200ml of milk and the other has 200ml of chocolate milk. They scoop 20ml of chocolate milk into the milk glass, then they scoop 20ml of whatever is in the milk glass into the chocolate milk glass.

When they’re done, is there more chocolate milk in the milk glass or more milk in the chocolate milk glass?

Difficulty - Easy, Interview Brain Teasers, Logic Puzzles

Number Deduction

A teacher gives three clever students in her class a challenge: she writes down 3 different numbers on 3 index cards, and has each student hold up one of the cards to their forehead such that they can’t see their own card but everyone else can.

She tells them each card has a different number, and that two of the numbers add up to the third number, and asks them to figure out their number without sharing the numbers they see.

Ava sees Sid has 40 on his forehead and Vlad has 60 on his forehead.

Ava says “I don’t know my number.”

Vlad says “I don’t know my number.”

Before Sid can say anything, Ava realizes she is now able to figure our her number! What is Ava’s number?

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Difficulty - Easy, Matchstick Puzzles, Simple Math

Matchstick Math 1

Move exactly 1 match to make this a valid equation.

Bonus: find two different ways to solve this.

In matchstick math puzzles, you are not allowed to make an inequality symbol such as ≠, ≥, >, <, or ≤. That would make it too easy!

Difficulty - Medium, Simple Math

Calculator Error Riddle

A kid is adding consecutive integers on a calculator, one at a time, starting with 1 + 2 + 3 + … and so on. At one point you notice the sum is now 100, but that shouldn’t be possible if the kid was adding this way. The kid tells you that he made an error and subtracted exactly one of the numbers he was supposed to add.

What is the number he subtracted?

Cryptography and Codes, Difficulty - Very Hard, Trivia and Knowledge, Word Puzzles

Tricky, Tricky

Can you figure out the number I am thinking of?

Some cryptic clues:

Call it sin, and mess it up (9)
Between θ and κ (4)
Engage emotionally (5)
Half a byte (6)
Mottle (5)
Never mix meds & gin (7)
– (4)

If you have all the answers right, that’s only about 33% percent of the solution: you need to justify them too.

Some clues were left over, so here’s a second set:

Sing about the matter (10)
Frog’s pole (3)
Heroin or slap (5)
0 or 1 (1,3)
A toy asking a question on an online forum (4+2 = 6)

An ingenious and very difficult puzzle by Bass on Puzzling StackExchange (spoiler warning: contains the solution)

View Solution

Difficulty - Medium, Word Puzzles

Unique Number of Letters

Out of all whole numbers between one and five thousand, there is only one number that has a unique number of letters when you spell it out. What number is it?

Include spaces and dashes. For example, “twenty-four” and “two hundred” both have 11 letters.

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Difficulty - Medium, Probability

How Many Daughters

A mother randomly selected two of her children to pick up a package from the post office. There was a 50% chance both children were daughters. How many daughters does this mother most likely have?