The Fundamental Counting Principle states that if an event can be chosen in p different ways and another independent event can be chosen in q different ways, the number of different arrangements of the events is p x q.
Addition rule of probability
The Additive Rule of Probability states that the union of two events can be found by adding the probabilities of each event and subtracting the intersection of the two events
Combinations are distinct arrangements of a specified number of objects without regard to order of selection from a specified set.
A permutation is an arrangement of objects where order is important.