First, you have a 5/10, or 1/2 chance of getting a blue.
Then, for red, it would be 3/9, or 1/3, because you already drew a marble out.
Finally, green would be 2/8, or 1/4, because by now you've already drawn out 2 marbles.
Multiply each of the chances: 1/2 * 1/3 * 1/4
The answer will be 1/24