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
