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

In a school, there are four subjects. Seventy percent of students study English, seventy five percent of students study Science, eighty five percent of students study Mathematics and eighty percent of students study Spanish.

Can you calculate the percentage of students that study all four subjects?

There is a five-letter word which has 3 consonants(all are same) and 2 different vowels.
Also, something wrong is associated with the word.

Can you name it?

I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I?

If an electric train is travelling south, then which way is the smoke going?

In the below-given picture of a pretty girl, there is something wrong. Can you find out what it is?

You walk into a room where there are three primates held in their respective cages:
1) A lion who is eating the flesh of a goat.
2) An orangutan who is playing with blocks.
3) A donkey who is sitting idle.

Who is the most intelligent primate in the room?

I know a ten-letter word in the English language which can be typed using only the top rows of the computer keyboard.

Can you find the missing number in the third row?
35 20 14
27 12 18
5 2 ?

Why is the Hole below a Lock?