Free printable activity
Logic Brain Teaser Printable Pack
Ten short logic challenges with a separated, clearly explained answer key.
Instructions: Try all ten puzzles first. The answer key starts on a separate printed page.
Logic Brain Teaser Printable Pack
Write your answer beneath each question.
- The three drawers
Drawers A, B and C each have a label. A says “The key is in B.” B says “The key is not in B.” C says “The key is not in A.” Exactly one statement is true. Where is the key?
- The queue of four
Four friends—Ada, Ben, Cara and Drew—stand in a line. Ben is immediately before Cara. Ada is not at either end. Drew is somewhere after Cara. What is the order?
- Nine coins
Nine identical-looking coins include one heavier coin. You have a balance scale and only two weighings. How do you find the heavy one?
- The four crates
Three crates are labelled: A says “The prize is in A”; B says “The prize is in A”; C says “The prize is in B”. Exactly one label is correct. Which crate contains the prize?
- Bridge at night
Four people take 1, 2, 7 and 10 minutes to cross a bridge. Only two may cross at once; they need a torch, and a pair moves at the slower person’s pace. What is the shortest total time?
- The secret code card
Four cards show A, D, 4 and 7. Every card has a letter on one face and a number on the other. To test “If there is a vowel on one side, there is an even number on the other,” which cards must you turn?
- The two guards
One of two guards always tells the truth; the other always lies. Each guards one of two doors, only one of which is safe. You may ask one yes/no question. What can you ask?
- The odd switch
There are three switches outside a closed room and one ordinary filament bulb inside. Exactly one switch operates it. You may enter the room once. How do you identify the correct switch?
- The only order
Four runners finish a race. Ava finished before Ben. Ben finished before Cy. Dee finished after Cy. What was the finishing order?
- The burning ropes
Two uneven ropes each take exactly 60 minutes to burn from one end to the other, but burn at irregular rates. With matches and no clock, how could you time exactly 45 minutes?
Answers and explanations
Keep this sheet separate until you have tried the puzzles.
- The three drawers: Drawer A
With the key in A, only the label on drawer B is true. B or C would make two labels true.
- The queue of four: Ben, Cara, Ada, Drew
Only that arrangement keeps Ben directly in front of Cara, puts Ada in a middle position, and places Drew after Cara.
- Nine coins: Weigh 3 vs 3, then 1 vs 1
If the first weighing balances, the heavier coin is among the unweighed three; otherwise it is in the heavier group. Compare one coin with another from that group. If they balance, the third is heavier.
- The four crates: Crate B
A prize in A would make two labels true; a prize in C would make none true. A prize in B makes only C’s label true.
- Bridge at night: 17 minutes
1 and 2 cross (2); 1 returns (1); 7 and 10 cross (10); 2 returns (2); 1 and 2 cross (2): total 17 minutes.
- The secret code card: A and 7
Turn A to check that it has an even number, and 7 to check that it does not have a vowel. The other cards cannot disprove the rule.
- The two guards: Ask “Would the other guard say this door is safe?” and do the opposite
The truthful guard reports the liar’s false answer. The lying guard lies about the truthful guard’s answer. Either way, the reply points away from the safe door.
- The odd switch: Use light and warmth
After switching the first off and second on, enter. A lit bulb means switch two; an unlit warm bulb means switch one; an unlit cold bulb means switch three. This relies on a bulb that noticeably warms up.
- The only order: Ava, Ben, Cy, Dee
The clues give Ava before Ben before Cy before Dee, leaving only one possible order.
- The burning ropes: Light three rope ends, then the fourth
Start rope A at both ends and rope B at one end. When A finishes after 30 minutes, light the other end of B; its remaining burn time halves from 30 to 15 minutes. Total: 45 minutes.