• Brain Teasers & Puzzles

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    • Calculate below as fast as you can, using only mental arithmetic (Without pen, paper, or a calculator!).

      Take 1000 and add 40.
      Add 1000.
      Add 30 and then add 1000.
      Add 20.
      Add 1000 and then add 10.

      What is the result of this calculation?

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    • A patient has fallen very ill and has been advised to take exactly one pill of medicine X and exactly one pill of medicine Y each day, lest he die from either illness or over dosage. These pills must be taken together. The patient has bottles of X pills and Y pills. He puts one of the X pills in his hand. Then while tilting the bottle of Y pills, two Y pills accidentally fall out. Now there are three pills in his hand. Because both types of pill look identical, he cannot tell which two pills are type Y and which is type X. Since the pills are extremely expensive, the patient does not wish to throw away the ones in his hand. How can he save the pills in his hand and still maintain a proper daily dosage?

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    • You are sitting with one opponent at an empty, round table. Taking turns, you should place one euro on the table, in such a way that it touches none of the coins that are already on the table. The first player that is not able to place a euro on the table has lost. By tossing a coin, it has been decided that you may start.

      Which strategy will you follow to make sure you are guaranteed to win?

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    • You walk into a room in which there are three primates: a chimpanzee, an orangutan, and a gorilla. The chimpanzee is holding a banana in each hand, the orangutan is holding a big stick, and the gorilla is holding nothing. Which primate in the room is the smartest?

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    • Four men want to cross a bridge. They all begin on the same side. It is night and they have only one flashlight with them. At most two men can cross the bridge at a time and any party who crosses, either one or two people, must have the flashlight with them. The flashlight must be walked back and forth: it cannot be thrown, etc. Each man walks at a different speed. A pair must walk together at the speed of the slower man. Man 1 needs 1 minute to cross the bridge, man 2 needs 2 minutes, man 3 needs 5 minutes, and man 4 needs 10 minutes. For example, if man 1 and man 3 walk across together, they need 5 minutes.

      How can all four men cross the bridge in 17 minutes?

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    • There is a tree 20 feet high, with a circumference of 3 feet. A vine starts at the base of the tree and winds around the tree 7 times before reaching the top. How long is the vine?

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    • Assuming year 1 started on Sunday, it can be shown that only some days are possible as a century’s final day. What are these days?

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    • Here you see a pool with six happy frogs. You must exchange the positions of the brown and green frogs, using the following rules:

      a frog can only jump to an empty water lily;
      a frog can only jump over at most one other frog, that has another color;
      a frog can only jump forward (so the green frogs jump right and the brown frogs jump left).

      How can this be done?

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    • There is a rich man living with two other people his butler and his maid. One day the rich man is sitting at his desk counting his money, preparing to deposit it at the bank. When he is done he goes to the bank, but when he arrives, he realises he left a $100 dollar bill on the desk. So he quickly called the house and told the butler that he forgot the $100 bill on the desk, and he will come home now to pick it up. When he arrived home he asked the butler what he did with the bill, the butler said he put it under the green book on the desk. When the rich man looked under the book, it wasn’t there, so he asked the maid if she saw it. She said she saw the $100 bill when she was dusting and put it between pages 67 and 68 of the green book. Right then and there the rich man called the police, and knew who stole it, even before the rich man checked inside the book to see if the $100 bill was there. How did he know?

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    • Vicky caught four animals in the Great Animal Forest and put them into four separate cages. The four animals are:

      the lion, that always tells the truth;
      the jackal, that always lies;
      the parrot, that always repeats the last given answer (if he is the first one, he randomly says “yes” or “no”);
      the giraffe, that is so slow that he always truthfully answers the previous question (if he is the first one, he also randomly says “yes” or “no”).
      The other animals want to free them and send the small, smart hedgehog on the way to size up the situation. In the middle of the night, the hedgehog creeps into the camp of the Vicky and tries to figure out which animal sits in which cage. One by one, he asks them “are you the jackal?” After hearing the four answers, the hedgehog only knows in which cage the giraffe sits. Then the hedgehog asks them in the same order “are you the giraffe?” After hearing the four answers, the hedgehog also knows in which cage the jackal sits. Because the hedgehog then still does not know in which cages all animals sit, he asks the first animal “are you the parrot?” The answer is “yes”, and now the hedgehog knows exactly which animal sits in which cage. The hedgehog returns to the other animals and together they free the captured animals.

      Which animal was in which cage?

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