Logic Puzzles

Classic logic puzzles in which you infer or deduce actions and outcomes based on information provided

Difficulty - Hard, Interview Brain Teasers, Logic Puzzles

Pirate Survivor Puzzle

The democratic pirates are at it again! Since last time, they have become very successful and have now expanded to 100 pirates. They decide this is too large of a group for plundering, so true to their democratic roots, they want to settle it with a vote:

They vote on whether to kick out the newest (least senior) pirate.
If a majority votes “aye”, then the newest pirate is kicked out, and the process repeats with the remaining pirates.
If half or more of the remaining pirates vote “nay”, then the vote is over and everyone remaining is safe.

Each pirate wants to stay in the group, but if that is assured, they would prefer to kick out as many other pirates as possible (fewer ways to split the treasure).

How many pirates will remain at the end of this process?

Difficulty - Hard, Interview Brain Teasers, Logic Puzzles

Handshake Puzzle

Hasan and Lauren attended a dinner party with 4 other couples. Since some people already knew some of the other guests, every person at the dinner party shook hands with every person they had not met before.

Lauren noticed that everyone else (excluding Lauren herself) ended up with a different number of handshakes!

Can you figure out how many people Hasan shook hands with?

Difficulty - Very Easy, Logic Puzzles

Logical List Puzzle

Which logical statement(s) below must be correct?

Exactly one statement on this list is incorrect
Exactly two statements on this list are incorrect
Exactly three statements on this list are incorrect
Exactly four statements on this list are incorrect
Exactly five statements on this list are incorrect
Exactly six statements on this list are incorrect
Exactly seven statements on this list are incorrect
Exactly eight statements on this list are incorrect
Exactly nine statements on this list are incorrect

Difficulty - Very Hard, Interview Brain Teasers, Logic Puzzles

Majority Vote Puzzle

After the results of an election, you are told that one candidate has received the majority of the votes, but you don’t know which candidate. You have exactly one opportunity to hear the votes, but:

The list of votes is very long
The votes will be announced one after another in random order
You have a poor memory (can only remember a couple of names or numbers)
You are not allowed to write or record anything

Given these restrictions, is there a way figure out which candidate received the majority of votes?

Difficulty - Hard, Logic Puzzles

Tennis Match Mystery

Abe, Ben, and Catelyn were playing 1-on-1 tennis matches one afternoon. After each match, the winner stayed on the court and the loser was replaced by the person who sat out.

At the end of their session, Abe had played 8 matches, Ben had played 12 matches, and Catelyn had played 14 matches. Ben was particularly exhausted as he had played all of the last 7 matches.

Is it possible to figure out who played and who won in the 4th match?

Difficulty - Medium, Interview Brain Teasers, Logic Puzzles, Number Theory

Deducing Bicycle Spokes

A bicycle shop has some number of unusual bicycles. All their bikes are identical (there’s more than one bike), and each bike’s front and back wheels has at least one spoke each (each bike has more than one spoke). You don’t know how many bicycles are in the shop, but you know there are between 200 and 300 spokes in total.

If you knew the exact number of spokes, you would be able to figure out the number of bicycles.

You don’t know the exact number of spokes, but just knowing the fact that you would be able to figure it out with that information allows you to deduce the answer. How many bicycles and spokes are there?

Difficulty - Easy, Interview Brain Teasers, Logic Puzzles

Guess the Playing Cards

There are three playing cards in a row.

There is a heart to the left of a diamond. There is a five to the right of a jack. There is a club to the left of a diamond. There is a queen to the left of a club.

What are the three cards?

Difficulty - Hard, Interview Brain Teasers, Logic Puzzles

5 Pirates Puzzle

Five pirates are figuring out how to divide up a newly plundered treasure of 100 gold coins. From most senior to least senior: Pirate A, Pirate B, Pirate C, Pirate D, and Pirate E. The rules:

The most senior pirate must propose a distribution (for example, giving himself all 100 gold coins and 0 for everyone else), and then all the pirates must vote on it.
If at least half of the pirates vote for a proposal, the proposal is accepted and the gold is split according to that distribution.
Otherwise, if the proposal is rejected, the pirate that proposed it is kicked out, and this process repeats with the remaining pirates.
Each pirate’s main goal is to maximize the gold they receive (but a pirate that is kicked out gets no gold), but are also spiteful enough to prefer kicking out the other pirates, all else equal. The pirates are also distrustful of each other and will not make any side deals, so the distribution of an accepted proposal is final.

What is the greatest number of coins that Pirate A can distribute to himself?

This is a classic logic puzzle, occasionally encountered in tech interviews many years ago.

Preparing for a brain teaser interview? Check out our ultimate guide to brain teaser interviews.

Difficulty - Easy, Game Theory and Game Strategies, Interview Brain Teasers, Logic Puzzles

Rigging a Vote

There are five candidates participating in an unusual election. There are four rounds of voting, and only two candidates participate in each round, with the winner moving on and the loser dropping out. This means first candidate 1 will run against candidate 2, then the winner will run against candidate 3, and so on, with the winner of the last round being elected.

Polling shows that Stephen Hawking is not very popular. There are five equal-sized groups in the voting population, and polling shows they prefer the candidates in the following order:

Group 1: Einstein > Newton > Maxwell > Hawking > Curie
Group 2: Newton > Curie > Maxwell > Einstein > Hawking
Group 3: Curie > Maxwell > Einstein > Newton > Hawking
Group 4: Einstein > Newton > Hawking > Curie > Maxwell
Group 5: Newton > Maxwell > Einstein > Hawking > Curie

But Hawking has a secret weapon – he gets to choose the order the candidates participate in each round.

Is there a way for Hawking to organize the rounds such that he wins?

Difficulty - Hard, Interview Brain Teasers, Logic Puzzles

Island of Blue Eyes

There is an island known for its people with blue eyes, yet there is at least one green-eyed person on the island. No one knows the color of their own eyes, as there are no reflective surfaces on the island and discussion of eye color is forbidden, but they can see everyone else’s eye color. If any islander were to come to know that they do not have blue eyes, they would leave the island in shame before the next sunrise.

One day, an outsider visits the island and remarked how there was at least one islander with green eyes. Within the day, every islander had heard and understood this new information.

Assuming departures from the island are noticed by everyone by the next day, and assuming each islander is highly logical and is able to keep track of all other islanders’ eye colors and actions – what happens to the islanders and does it depend on the number of green-eyed islanders?

This is a classic logic puzzle, also known under a different story (but same core logic) as Josephine’s Problem.