There are 6 red marbles, 4 blue marbles, and 6 yellow marbles in a bag. Two marbles are chosen without replacing them in the bag. What is the probability that a red marble is chosen, followed by a blue marble?

This is a basic probability question. The two results are not independent of one another because, after selecting a red marble, you do not replace it, but then proceed to select a blue marble from the remainder.
(6 red) + (4 blue) + (6 yellow) = 16 marbles total.
Probability of first selecting a red marble = 6/16 = 3/8.
Probability of selecting a blue marble after one marble has already been removed = 4/15.
Probability of this particular sequence (red, then blue) = (3/8) x (4/15) =
12/120 = 6/60 = 3/30. The answer is 3/30.