A bag contains 7 red marbles, 5 white marbles, and 9 blue marbles. You draw 3 marbles out at random, without replacement. What is the probability that none of the marbles are red?

The sum of all the marbles is 21. There are 21 marbles to choose from, so the probability of the first marble not being red is 14/21.
Assuming that this marble is not red, we choose the second marble. This time the probability is 13/20, because we have removed a marble that is not red, and there are 20 marbles left in the bag.
The third probability is 12/19.
Last, multiply them all together.
14/21 * 13/20 * 12/19 = 26/95