# Category Briefly

In a basket we have 20 balls numbered from 1 to 20. How many balls must be extracted at least (without looking) to be completely sure that the sum of the numbers of the balls extracted is greater than or equal to 70? Solution The worst possible scenario would be to extract the balls with the lowest score.

Using three nines and with the symbols and mathematical operations you need, are you able to get the numbers from 1 to 12? Look at this example for zero: (9 - 9) × 9 = 0 Solution This watch contains the solution:
This is a problem to solve mentally, without the help of paper and pencil or other means. We have three boxes of equal size individual and separated from each other. Inside each box there are two other smaller ones and in each of these another four still smaller ones. How many boxes are there in total? Solution We have 33 boxes in total: 3 large boxes 6 medium boxes 24 small boxes
In the kingdom of ingenuity someone stole the crown of the prince who was about to be crowned king. The soldiers arrested three suspects among whom was the only one guilty of the robbery. They were brought before the prince and it happened that the first immediately accused the second of the robbery.
We are in a two-story house. On the ground floor there are three switches and on the upper floor there is a single bulb. How can we find out which of the three switches is the one that turns on that bulb taking into account that we can only go up one time to the floor above to check if the bulb is on?
I have three sisters who never recognize their age. When asked about age they always answer the following: 1) Alicia and Beatriz say that one of them is the oldest of the three. 2) One of two, or Carla is the oldest or Alicia is the youngest. Which is the largest of the three? Solution On the first track, if Alicia were the oldest, then on the 2nd track Carla could not be the same and Alicia would also be the smallest so we reached a contradiction.
A videotape can record 2 hours in high quality mode (SP) or 4 hours in medium quality mode (LP) or 6 hours in low quality mode (XLP). After recording a 32-minute chapter of my favorite series in SP mode and a 44-minute documentary in LP mode, how many minutes can still be recorded in XLP mode?
We know that there cannot be two consecutive prime numbers, except for the pair {2, 3}. This is obvious if we think that in any pair of consecutive numbers, one of them will be even. And the only even prime number is 2. Now we consider the following: are there two consecutive odd ones that are cousins? For example.
The operator of the telephone company has an arduous task. You must connect 300 cables to enable the telephone line to 300 houses in a new urbanization. The problem is that the telephone exchange where the cables come from is in a neighboring town 5km away without any means of communication and the operator has only one battery and one bulb as the only tools to identify the two ends of each cable and power make connections correctly.
A carpet of 8m by 5m was damaged so it was necessary to cut a rectangle in the center of 4m by 1m as seen in the figure. Someone came up with an ingenious method to cut the carpet in two parts with which you could build a square carpet 6 meters side. What did the two pieces look like?
Why do you see a cube in an image formed by circles separated from each other? Surely by the law of "good continuity" described by the Gestalt psychological school (German word that means "configuration" or "organized totality") This branch of psychology holds that the totalities are prior, both in perception and behavior , to the parts that compose it.
On a sunny morning a high-speed train from the RENFE company departed from the Zaragoza station bound for Barcelona. Who will travel faster, the train or its shadow? Solution In principle, the speed of the train is the same as that of its shadow, but if we take into account that the shadow of a static object moves from west to east we must assume that the shadow of the train will travel more slowly when the train moves towards the West but faster when the train travels east.
If today is Sunday, what is the day that follows the day that comes after the day that precedes the day before yesterday? Solution It's Saturday.
It may sound somewhat paradoxical, but we are going to crumble the foundations of mathematics by showing that 1 + 1 is equal to 3. We begin with an unquestionable equality: We add both members of equality the same amount: We can transform the previous expression into: If we We fix, we see that we have on both sides the result of squareing a binomial, so we can simplify: If we extract the square root of both members we have that: Or what is the same Or that: How is it possible?
Anamorphosis is a reversible deformation of an image produced by an optical procedure (such as using a curved mirror) or through a mathematical procedure. It is a perspective effect used in art to force the observer to a certain preset or privileged point of view, from which the element takes a proportionate and clear form.
Two masons are divided into two parts a lot of 100 bricks. As the work is very late, they do not want to waste time counting bricks so they distribute them by eye so that each one has more or less half. The first mason has to place them in rows of 5 bricks while the second places them in columns of 7 bricks.
A sheikh has to transport 100 gold bars of 1 kilo of weight each. For this it has 10 camels and 1 guard for each camel. Each of these camels carries 10 ingots. At the end of the trip, the sheikh's confidant tells him that one of the guards has stolen 1 gram of gold from each of the 10 bullion that the guard was carrying but does not know with certainty what guard he is.
Andrés, Benjamín and Carlos are three friends who love the QuickClick game, which consists of 40 clicks with the computer mouse at the fastest possible speed. Andrés is able to click 20 times in 20 seconds, Benjamin clicks 10 times in 10 seconds and Carlos clicks 5 times in five seconds.
A train leaves the station with half of the seats occupied. In the first stop as many people went up as they went down. In the second stop, half of the people who got off at the first stop went up and twice as many people went up at the first stop. As many people as the sum of the people who got off at the first two stops went up on the last stop and as many people went down as the sum of the people who got on the first two stops.
Buffalo Bill and Butch Cassidy are fighting for the showgirl's favors. At one point, Butch proposes to Bill to play Russian roulette. They take a revolver of 6 bullets and leave only three bullets housed in consecutive chambers. Bill takes the revolver without thinking and shoots at his own temple but fortunately the camera did not contain any bullet so now it is Butch's turn.
My uncle Joaquin has a disease that forces him to take one pill of each of the two different medicines that the doctor prescribed for 30 consecutive days. The pharmacist gave him a bottle of medicine "A" and a bottle of medicine "B" each of which contained exactly 30 pills.