Fundamental principle of counting: If task A can be done in m ways and task B in n ways, both can be done in m × n ways. Extends to multiple tasks: m × n × p × ... Arrangement problems: Count different orderings satisfying conditions. Example: Number of 3-digit numbers using digits 1-9 with repetition: First digit (1-9) = 9 ways, second = 9 ways, third = 9 ways. Total = 9 × 9 × 9 = 729. Without repetition: 9 × 8 × 7 = 504. Solving MCQs: Break task into independent steps; multiply count for each. Shortcut: Identify whether order matters, whether repetition allowed. Exam tip: Use counting principle before permutation formulas. Verify by listing small cases.