Use the multiplication principle: for each shirt there are 5 pant choices and 1 shoe choices. Total outfits = 4 × 5 × 1 = 20. The fundamental counting principle says to multiply the number of choices at each stage. Each shirt choice can pair with every pant choice and every shoe choice. Adding would count only separate choices, but multiplying counts complete combinations. The correct answer is 20, which matches the calculation or reasoning shown.