Math riddles and brain teasers, with answers

I spent a whole night trying to solve the light bulb puzzle. By the time I got it, the sun was up and I had eaten an entire bamboo grove. Worth it.

Here are 24 brain teasers, from easiest to hardest, split into logic, math and lateral thinking. Try to answer before reading the solution right after each question.

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Logic puzzles

1. A farmer must cross a river with a wolf, a goat and a cabbage. The boat holds him and one more thing. Left alone, the wolf eats the goat and the goat eats the cabbage. How does he do it? Answer: take the goat over and return; take the wolf and bring the goat back; take the cabbage and return; finally, take the goat.
2. Three switches outside a closed room, and only one turns on the bulb inside. You can enter only once. How do you find the right one? Answer: turn on the first switch for a few minutes, then off; turn on the second and go in. If the bulb is on, it is the second; off and warm, the first; off and cold, the third.
3. You have a 3-liter jug and a 5-liter jug. How do you measure exactly 4 liters? Answer: fill the 5 and pour into the 3 (2 left in the 5). Empty the 3 and pour the 2 liters in. Fill the 5 again and top up the 3 (only 1 liter fits). 4 liters are left in the 5.
4. Two doors, two guards: one always lies, one always tells the truth, and you do not know which is which. One door leads out. With one question, how do you find the exit? Answer: ask either guard: "If I asked the other guard which door is the exit, what would he say?" Then take the other door.
5. You have 8 identical balls, but one is slightly heavier. With a balance scale, how do you find it in just two weighings? Answer: weigh 3 against 3. If they balance, the heavy one is among the other 2: weigh them against each other. If not, take the heavier 3 and weigh 1 against 1; if those balance, it is the third.
6. Two ropes each burn in exactly 1 hour, but unevenly. How do you measure 45 minutes? Answer: light the first rope at both ends and the second at one end. When the first burns out (30 minutes), light the other end of the second. It burns out 15 minutes later.
7. In a race, you pass the person in second place. What place are you in? Answer: second.
8. A man looks at a photo and says: "Brothers and sisters I have none, but that man's father is my father's son." Who is in the photo? Answer: his son.

Math riddles

1. A bat and a ball cost $1.10 together. The bat costs $1.00 more than the ball. How much is the ball? Answer: 5 cents. If the ball were 10 cents, the bat would be $1.10 and the total $1.20.
2. If 5 machines make 5 widgets in 5 minutes, how long do 100 machines take to make 100 widgets? Answer: 5 minutes. Each machine makes one widget in 5 minutes.
3. A patch of lily pads doubles in size every day and covers the whole lake in 48 days. When does it cover half the lake? Answer: on day 47.
4. Divide 30 by half and add 10. What do you get? Answer: 70. Dividing by half is the same as multiplying by 2.
5. A brick weighs 1 kg plus half a brick. How much does the whole brick weigh? Answer: 2 kg.
6. How many times can you subtract 5 from 25? Answer: once. After that, you are subtracting from 20.
7. A father is 36 and his son is 6. In how many years will the father be twice as old as his son? Answer: 24 years: the father will be 60 and the son 30.
8. Add in your head: 1000 + 40 + 1000 + 30 + 1000 + 20 + 1000 + 10. Answer: 4,100. Many people say 5,000.
9. What comes next: 1, 1, 2, 3, 5, 8, ...? Answer: 13. Each number is the sum of the two before it.
10. What letter comes next: O, T, T, F, F, S, S, E, ...? Answer: N, for nine. They are the first letters of the numbers.

Lateral thinking puzzles

1. How many months have 28 days? Answer: all 12.
2. A plane crashes right on the border between two countries. Where do they bury the survivors? Answer: nowhere: you do not bury survivors.
3. How many animals of each kind did Moses take on the ark? Answer: none. It was Noah.
4. A man lives on the 10th floor. Every morning he takes the elevator down. Coming home, he rides only to the 7th floor and walks the rest, except on rainy days. Why? Answer: he is short and can only reach the button for 7. On rainy days, he presses 10 with his umbrella.
5. A woman shoots her husband, holds him under water for five minutes, then hangs him. Later they have dinner together. How? Answer: she is a photographer: she shot a photo of him, developed it in water and hung it up to dry.
6. On a game show you pick one of three doors; one hides a prize. The host, who knows where it is, opens one of the other two, which is empty, and asks if you want to switch. Should you? Answer: yes. Switching wins 2 times out of 3. This is the famous Monty Hall problem.

Train your brain by playing

If you like brain teasers, you will like puzzle games. In Gridspark, you place colorful blocks to clear rows and columns, and every move has to leave room for the next one.

PolarzinhoIn Gridspark, the riddle is always the same: where does this piece fit?

And to test your knowledge as a group, Zabum! is a general knowledge quiz with a live scoreboard and rooms to play with friends.

PolarzinhoAfter 24 brain teasers, I thought Zabum! would be easy. It was not.

FAQ

What is the hardest brain teaser on this list?

The 8 balls and the two burning ropes usually take the longest.

What is the difference between a riddle and a brain teaser?

A riddle is usually a guessing game with wordplay. A brain teaser asks for reasoning: logic, math or thinking outside the box.

What is a lateral thinking puzzle?

A puzzle where the obvious reading leads you the wrong way, and the answer comes from looking at the situation from a different angle.

Your turn

Pick three puzzles and try them on someone at home without showing the answers. Then challenge your brain in the games below.

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