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

If a shopkeeper can only place the weights on one side of the common balance. For example, if he has weights 1 and 3 then he can measure 1, 3 and 4 only. Now the question is how many minimum weights and names of the weights you will need to measure all weights from 1 to 1000? This is a fairly simple problem and very easy to prove also.

There is an English word that can be used up to four times in a row without modifying the spelling and form a valid grammatical sentence.

Do you know what word is that?

Christina has four daughters, and each of her daughters has a brother. How many children does Christina have?

Which three-digit number, made of consecutive digits, like 567, is 2 less than a cube and 2 more than a square?

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

There is a box in which distinct numbered balls have been kept. You have to pick two balls randomly from the lot.

If someone is offering you a 2 to 1 odds that the numbers will be relatively prime, for example
If the balls you picked had the numbers 6 and 13, you lose $1.
If the balls you picked had the numbers 5 and 25, you win $2.

Will you accept that bet?

Find the next number in the series.
1 10 24 43 67 ?

I live in a bowl. I can swim. I have a tail. I also have fins and big eyes. What am I?

There is one thing that goes round the house and also inside the house but never even share a brief touch. What is it?