· Brain teasers
10 Brain Teasers for a 5-Minute Break (with Answers)
Ten short logic puzzles you can solve over a coffee, from easy to tricky. The answers stay hidden until you want them.
Got five minutes? Here are ten classic brain teasers, roughly from easiest to trickiest. Each one fits on a napkin and needs nothing harder than simple arithmetic. Try each one before you open its answer: the moment it clicks is the fun part.
1. The bat and the ball
A bat and a ball cost $1.10 together. The bat costs $1.00 more than the ball. How much does the ball cost?
Show the answer
5 cents. The quick answer, 10 cents, doesn't work: the bat would cost $1.10 and the total would be $1.20. With a 5-cent ball, the bat costs $1.05 and together they make $1.10.
2. The lily pads
A patch of lily pads doubles in size every day. After 48 days it covers the whole lake. On which day did it cover half the lake?
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Day 47. It doubles every day, so the day before it covered the whole lake, it covered half.
3. What comes next?
1, 11, 21, 1211, 111221, …
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312211. Each line reads the one before it out loud: "1" is one 1 (11), "11" is two 1s (21), "21" is one 2, one 1 (1211), and so on. 111221 is three 1s, two 2s, one 1: 312211.
4. Three switches, one bulb
Three switches outside a closed room control a single old-style light bulb inside. You can flip the switches as much as you like, but you may open the door only once. How do you find which switch works the bulb?
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Turn the first switch on and wait a few minutes. Turn it off, turn the second one on, and open the door. If the bulb is on, it's the second switch. If it's off but warm, it's the first. If it's off and cold, it's the third.
5. The mislabelled boxes
Three boxes are labelled Apples, Oranges and Apples & Oranges. Every label is wrong. You may take one fruit from one box, without looking inside. How do you fix all three labels?
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Take a fruit from the box labelled Apples & Oranges. Its label is wrong, so it holds only one kind: the fruit you pick tells you which. Say it's an apple: that box is Apples. The box labelled Oranges can't be oranges (wrong label) and can't be apples (found already), so it's Apples & Oranges. The last box is Oranges.
6. Four litres
You have a 5-litre jug, a 3-litre jug and a tap. Neither jug has markings. How do you measure exactly 4 litres?
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Fill the 5-litre jug and pour it into the 3-litre jug: 2 litres stay in the big one. Empty the small jug and pour the 2 litres into it. Fill the 5-litre jug again and top up the small jug, which needs exactly 1 litre. 4 litres are left in the big jug.
7. The heavy coin
You have 9 coins that look the same. One is slightly heavier. With a balance scale, how do you find it in just two weighings?
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Split the coins into three groups of 3 and weigh two groups against each other. If one side is heavier, the coin is in that group; if they balance, it's in the group you left out. Then weigh two coins from that group: the heavier one is the coin, and if they balance, it's the third.
8. The bridge at night
Four friends must cross a narrow bridge at night with one torch. At most two can cross at once, and anyone crossing needs the torch. They take 1, 2, 5 and 10 minutes to cross, and a pair walks at the slower one's pace. What's the shortest time to get everyone across?
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17 minutes. The 1 and the 2 cross (2 minutes). The 1 brings the torch back (1). The 5 and the 10 cross together (10). The 2 brings the torch back (2). The 1 and the 2 cross again (2). The trick is sending the two slowest across together.
9. Fifteen minutes
You have two sand timers: one runs for 7 minutes, the other for 11. How do you time exactly 15 minutes?
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Start both. When the 7-minute timer runs out, turn it over. When the 11-minute timer runs out (11 minutes in), the 7-minute timer has 4 minutes of sand at the bottom: turn it over again, and when it runs out, 15 minutes have passed.
10. Three arrows in a row
A board is a single row of three squares, with an arrow on each. From left to right they point right, right, left. Tapping an arrow slides it off the board in its direction, but only if nothing is in its way. In what order can you clear them?
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You can't. The middle arrow points right, straight at the third arrow, and the third points left, straight back at it. Each blocks the other, so neither can ever leave, and the first arrow is stuck behind them. That's the one trap an arrow puzzle designer has to avoid.
How did you do?
If a few of these took you a while, that's the point: a teaser that's slow to crack is the one you remember. For ways to get faster, read how to get better at logic puzzles.
