Below, you will find the mathematical proof that 10 equals 9.99999?. But is that possible or there is something wrong about it? Can you find the error?

x = 9.999999...
10x = 99.999999...
10x - x = 90
9x = 90
x = 10

Place the numbers from 1 to 9 in the circles in a manner that each side of the triangle sums up to 17.

In the figure that has been attached to this question, each digit represents a digit. The similar letters carry the same integer value. Can you expose the original digits?

You are given four results as below:
1111 = F
2222 = E
3333 = T
Then, can you find out how to code 4444?

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By including only four matchsticks in below picture, you have to make "Four Triangles And Two Squares"

Find the Goldfish below the given Image.

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Look at the picture and find out which square is not linked with any of the other squares.

What building has the most stories?