Answer:

Explanation:
Let's separate into 3 cases:
Note that we put the number of color balls we want in the numerator and the total number of color balls remaining in the denominator.
Case 1: 2 blue marbles
(There were 5 blues originally, and 4 blues after 1 blue is taken out, same logic but different colors and values in case 2 and 3.)
Case 2: 2 red marbles

Case 3: 2 green marbles

Now add up all 3 cases and you will have the probability as required.
