I am white when I am dirty, and black when I am clean. What am I?

Three cars are driving on a track that forms a perfect circle and is wide enough that multiple cars can pass anytime. The car that is leading in the race right now is driving at 55 MPH and the car that is trailing at the last is going at 45 MPH. The car that is in the middle is somewhere between these two speeds.
Right now, you can assume that there is a distance of x miles between the leading car and the middle car and x miles between the middle car and the last car and also, x is not equal to 0 or 1.
The cars maintain their speed till the leading car catches up with the last car and then every car stops. In this scenario, do you think of any point when the distance between any two pairs will again be x miles i.e. the pairs will be x distance apart at the same time ?

Look at the below sequence of letters and tell us what the next letter should be:

O T T F F S S ?

Which five letter word can be added at the end of below word to form a
valid word?

> Pin
> Check
> Boiling
> View

What happened when the wheel was invented?

Can you replace the question mark with the correct number in the below table sequence?

What do the following words have in common?

Assess
Banana
Dresser
Grammar
Potato
Revive
Uneven
Voodoo

There are a number of books on a shelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?

You are stuck on an island where you have nothing. You find four pieces of paper somewhere on the island. What will be your strategy to escape from the island safely?

PS: No other resource is available to you on the island, neither can you build anything.

I am never scared but became petrified and can’t live in a house but would die to make one. What am I?