Question #2: There are 4 red, 5 yellow and 6 blue, for a total of 15.
There are 15C3 to choose 3 marbles from 15.
To get 2 red, 1 yellow, 0 blue: 4C2 x 5C1 x 6C0
To get 2 red, 0 yellow, 1 blue: 4C2 x 5C0 x 6C1
To get 1 red, 2 yellow, 0 blue: 4C1 x 5C2 x 6C0
To get 0 red, 2 yellow, 1 blue: 4C0 x 5C2 x 6C1
To get 1 red, 0 yellow, 2 blue: 4C1 x 5C0 x 6C2
To get 0 red, 1 yellow, 2 blue: 4C0 x 5C1 x 6C2
Add all the individual possibilities for red, yellow, and blue, and divide by the total number possible.( 15C3 ).