Logic Puzzles
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50 free Logic Puzzles trivia questions with answers — trivia quiz, new questions added Oct 2026.
A good logic puzzle needs no special knowledge, only a clear head. The trap is that the first answer to pop into your mind is often wrong, which is why researchers use questions like these to measure how carefully people think before they reply. This quiz mixes puzzles you solve on the spot with questions about the most famous brain teasers ever set. You will ferry awkward passengers across a river, weigh suspect coins, continue number sequences, judge the odds of a coin toss and work out who is lying. Along the way you will meet the game shows, board games, video games and puzzle setters that turned logical thinking into entertainment. Every answer comes with a short explanation of the reasoning, so a wrong guess still teaches the trick. It suits anyone who enjoys a logical quiz: puzzle fans, teachers looking for a lesson warm-up, quiz hosts, and job hunters practising for interview brain teasers.
30 of 50 questions with answers and explanations. Play the quiz
Q 01A 100 kg load of potatoes that is 99% water dries out until it is 98% water; what does it weigh now?
50 kg
50 kg is all that remains: the 1 kg of solid potato must now be 2% of the load rather than 1%, so the total weight has to halve.
Q 02In the film WarGames, which simple game does a computer play against itself to learn that nuclear war cannot be won?
Tic-tac-toe
Tic-tac-toe always ends level when both sides play perfectly, so the computer's endless draws teach it that some games have no winner.
Q 03A drawer holds loose black socks and blue socks; without looking, how few must you pull to guarantee a matching pair?
Three
Three is enough: with only two colours, the third sock must match one of the first two, a rule mathematicians call the pigeonhole principle.
Q 04How many people must gather in a room before it is more likely than not that two share a birthday?
23
23 people are enough, because a group that size already contains 253 different pairs, and any one of those pairs could be the match.
Q 05If 5 machines take 5 minutes to make 5 widgets, how many minutes would 100 machines need to make 100 widgets?
5
5 minutes is right, because each machine takes five minutes to build one widget, so a hundred machines working side by side finish a hundred in the same time.
Q 06In Tversky and Kahneman's test about Linda, an outspoken graduate, which description is logically the most probable?
A bank teller
A bank teller is the only safe pick: adding a detail can never make a description more likely, yet most people asked chose the feminist version.
Q 07A fair coin has landed heads twenty times in a row; what is the chance that the next toss is also heads?
1 in 2
1 in 2 is the chance on every single toss, because a coin has no memory; believing that tails is now overdue is known as the gambler's fallacy.
Q 08What is the greatest number of pieces a round pizza can be divided into using only four straight cuts?
11
11 pieces are possible, because a cut that crosses every earlier cut adds as many pieces as its own number: the fourth adds four to the seven made by three.
Q 09In Minesweeper, what does a number shown on an opened square tell the player about the squares touching it?
How many hide mines
How many hide mines is what the number reports, counting all eight neighbours, so each safe click is a small deduction and not a guess.
Q 10Water pouring puzzles, solved by tipping liquid between jugs of fixed sizes, are nicknamed after which action sequel?
Die Hard with a Vengeance
Die Hard with a Vengeance has a 1995 scene built on the puzzle, in which jugs holding three and five units must be used to measure out exactly four.
Q 11In the classic puzzle of ferrying a wolf, a goat and a cabbage one by one, how few river crossings are needed?
Seven
Seven crossings do it, four forward and three back, and the trick is that the goat rides back across once so it is never left alone with the wolf or the cabbage.
Q 12Which number comes next in the sequence 1, 1, 2, 3, 5, 8, 13?
21
21 follows, as 8 plus 13, in a pattern named after Fibonacci, the Italian whose book of 1202 used it to count a growing population of rabbits.
Q 13The Monty Hall problem, a three-door brain teaser, takes its name from the first host of which American game show?
Let's Make a Deal
Let's Make a Deal inspired the puzzle, in which switching doors wins two times in three; Monty Hall later demonstrated it to a reporter with a set of car keys.
Q 21Cards show 3, 8, blue and red; which two must you flip to test the rule 'even numbers have blue backs'?
8 and red
8 and red are the only cards that could break the rule, yet fewer than one person in ten chose them in Peter Wason's original study.
Q 22Each term of the sequence 1, 11, 21, 1211, 111221 reads the previous term aloud; which term comes next?
312211
312211 comes next, because the term before it reads as three 1s, two 2s, one 1; start from 22 instead and the sequence never changes at all.
Q 23Four walkers who need 1, 2, 5 and 8 minutes must cross a two-person bridge sharing one torch; how few minutes suffice?
15
15 minutes is the best possible, and the trick is to send the two slowest walkers over together, which beats the tempting plan of having the fastest escort everyone.
Q 14Which board game, devised by Anthony Pratt during wartime air raids, is won by ruling out suspects, weapons and rooms?
Cluedo
Cluedo, sold in North America as Clue, was first patented under the blunt name Murder; its title is a play on Ludo, a board game few Americans knew.
Q 15Which code-breaking board game, invented by an Israeli postmaster in 1970, scores every guess with small key pegs?
Mastermind
Mastermind is based on Bulls and Cows, a pencil-and-paper game that may date back a century, and it was shown to the big toy firms before a plastics company took it on.
Q 16What is the smallest number of colours that is always enough to colour any flat map so no bordering regions match?
Four
Four colours always suffice, as Francis Guthrie guessed in 1852 while colouring a map of England's counties; the proof arrived only in 1976, with a computer's help.
Q 17What is the most queens that can stand on a standard chessboard with no queen able to attack any other?
Eight
Eight is the limit, one queen for each row, and the puzzle of arranging them, first published in 1848, has exactly 92 solutions.
Q 18Which society, founded at Oxford in 1946, admits anyone scoring in the top two percent on an approved IQ test?
Mensa
Mensa takes its name from the Latin word for table, chosen to suggest a round table at which every member sits as an equal.
Q 19Which number comes next in the sequence 2, 3, 5, 7, 11, 13, 17, whose members divide only by themselves and one?
19
19 is the next prime, and the list never runs out: Euclid showed around 300 BC that there are infinitely many primes.
Q 20In which game-theory puzzle does each of two partners gain by betraying the other, though both do worse if both betray?
The prisoner's dilemma
The prisoner's dilemma was devised at the RAND Corporation in 1950, and when two colleagues played a hundred rounds they often chose to cooperate after all.
Q 24The classic puzzle of two uneven fuses that each burn for one minute asks solvers to time which exact interval?
45 seconds
45 seconds is the target: light both ends of one fuse and one end of the other, then light the last end when the first fuse dies after half a minute.
Q 25One of nine identical-looking coins is slightly light; how few weighings on a balance scale are sure to find it?
2
2 weighings always work: set three coins against three to learn which trio holds the light one, then compare two coins from that trio.
Q 26In a Tower of Hanoi, where no disc may sit on a smaller one, how few moves will shift five discs?
31
31 moves is the minimum, since each extra disc doubles the count and adds one; in the legend sold with the puzzle, priests moving 64 golden discs will finish as the world ends.
Q 27Why can 31 dominoes never tile a chessboard that has had two diagonally opposite corners cut away?
Both missing squares share a colour
Both missing squares share a colour, which leaves 32 squares of one colour and 30 of the other, while every domino has to cover one of each.
Q 28In the classic letter sum SEND + MORE = MONEY, where each letter hides a digit, which digit must M be?
1
1 is forced, because adding two four-digit numbers can carry no more than one into a fifth column; Henry Dudeney printed the puzzle in The Strand Magazine in 1924.
Q 29On an island of constant liars and constant truth-tellers, a native says 'my friend and I are both liars'; what follows?
Only the speaker lies
Only the speaker lies: no truth-teller could call himself a liar, so the claim is false, and it can be false only if the friend is honest.
Q 30Three children's ages multiply to 36 and add up to 13, and only one child is the eldest; what are the ages?
2, 2 and 9
2, 2 and 9 is the only fit: 1, 6 and 6 also makes 36 and 13, but then the eldest would be a twin, the clue that settles the classic census-taker puzzle.