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

What has many keys but can’t open a single lock?

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 ?

Can you find out who is the kidnapper in below Picture and why, Explain ?

It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

Your last good ping-pong ball fell into a narrow metal pipe embedded one foot deep in concrete.
How can you get it out undamaged, if all the tools you have are your tennis paddle, shoe laces, and plastic water bottle, which does not fit into the pipe?

What breaks yet never falls, and what falls yet never breaks?

A man in a car saw a Golden Door, Silver Door and a Bronze Door. What door did he open first?

Which number should replace the question mark?

I have holes on the top and bottom. I have holes on my left and on my right. And I have holes in the middle, yet I still hold water. What am I?