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

Kohli and Dhoni decided to have a paintball match.
Kohli told Dhoni, "Sir it is not right as you got thrice the paintballs I have".

Dhoni instantly gives Kohli 10 hit paintballs.
Kohli replied, "Sir, I believe you still have twice the paintballs than I have".

Dhoni gives "?" more paintballs to Kohli.
Kohli replied, "Sir it is even now, Let us have a match".

What is "?".

Take 9 from 6, 10 from 9, 50 from 40, and leave 6.

How Come ??

Which alphabet letter is a question?

I possess a head

I flaunt a tail

But I don't have mouth or legs

Can you find out who I am?

Irene killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

How do you go from 98 to 720 using just one letter?

There are 3 apples in the basket and you take away 2. How many apples do you have now?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

You need to move three matchsticks to form three squares. Can you do it?