posted by James on .
There are 5 red and 4 black balls in a box. If three balls are picked without replacement, what is the probability that at least one of them is red?
First, let's consider the probability that none of the balls are red, or all of them are blue. There are 9 balls, 4 of which are blue. So the chance that 3 blue balls will be chosen is 4/9 * 3/8 * 2/7, or 1/21. You can do the rest from here.