I roughly have the answers of these ones, but I can't find a correct way to explain everything...

We have a total of 15 balls in a box : 6 red, 5 green and 4 blue.

1) We assume that we pick balls one by one and any taken balls will be put back in the box. What is the probability of getting, in this order, a red ball, then a green, then a blue one ?

The way of doing is blurry to me, but I understood it this way : we take a ball and if it's not red, we put it back. And we continue as long as we get a red ball.

We keep this ball, and we go on.

The answer is something like $\displaystyle \frac{C_6^1 C_5^1 C_4^1}{C_{15}^3}$.