Brain Teasers and Puzzles

Brain teasers, puzzles, riddles, and other challenges

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)

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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?

Difficulty - Medium, Rebus

Rebus Puzzle 16

What common word or phrase is this rebus referring to?

     SIXTEEN
THOUSAND
   FIFTY
  NINE
     TRILLION
THIRTY

Solution

Safety in numbers

(“Safety” is spelled out downwards in the numbers listed.)

Difficulty - Hard, Interview Brain Teasers

Battery Test Puzzle

You have four good/new batteries and four bad/used batteries but don’t know which are which. You have a flashlight that uses two batteries, which will only work if both batteries are good – if it doesn’t work, you won’t know if both batteries are bad or just one.

How many pairs of batteries do you need to test to guarantee you find a good pair?