Abag has 7 red marbles, 8 blue marbles and 3 black marbles. what is the probability of picking a blue marble, replacing it, and then picking a black marble?

The number of sample space by which a marble can be picked without replacement can be calculated through the principles of counting.
                     test period = (15 x 14 x 13)
                                      = 2730

Using the basic principles of counting, we yield an answer of,
                      = (8)(3) = 24

The probability is therefore the quotient of these items.
                        Ratio = 24/2730
Simplifying,
                        Ratio = 4/455