This is a page that contains various riddles, questions and quizzes that I have collected over the years.
Don't feel embarrassed if you cannot solve them :-)

The difficulty is in a 10 scale.

1=2 [4]

x^2 =x+x+x+x+x+....     (x times)

Taking the derivative in both sides we get:

2x = 1+1+1+1+ ... (x times)

But since 1 + 1 + 1 + 1 ... (x times) = x

Then
2x = x
2 = 1

What is she doing now? [3]

In 3 years a mother will be 12 times older than her child.
In 9 years she will be 4 times older than her child.

4 4's [9]

With 4 4's and any mathematical symbol that you think it's useful, try to create the number 33.
For example, 32 = 4*( 4 + Sqrt[4]*Sqrt[4] ) , 31 = 4! + (4!+4)/4.

I first dealt with this problem in high school, where I managed to create all numbers from 1 to 100, except for 3-4 cases.
33 was the smallest number that I couldn't figure it out, despite days of thinking. Since then 33 is my favorite number.
I came across the solution years later, realizing that such games where purposeless since I could create any number I wanted.

This is the so-called endless triangle. Most people actually think that this is a trick, but it isn't. In fact, this is a real object and this is exactly how you would see it if it was just in front of you right now. The only thing is, this is the one and only viewing angle that this object looks that way! Can you draw it from another angle then?

Lock & Key [6]

You want to send a valuable object to a friend. You have a box which is more than large enough to contain the object. You have several locks with keys. The box has a locking ring which is more than large enough to have a lock attached. But your friend does not have the key to any lock that you have. Note that you cannot send a key in an unlocked box, since it might be copied.

Missing letter [8]

Which is the letter missing from the following sequence?

A A A A ? A A A A A A

Hint: The answer is not A.

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