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