There are 4 shirts and 3 pairs of jeans, so using the fundamental counting principle, the number of possible outfit combinations is 4 * 3 = 12. The fundamental counting principle states that to calculate the number of possible outcomes of a series of events, you multiply the number of possible outcomes of each individual event. Examples are given of using this principle to calculate the number of possible 3-course meal combinations from various options, ways to customize a car order, and possible answers to a 10-item multiple choice quiz. The document also calculates the number of possible 10-digit telephone numbers in the US.