How many 4-digit numbers can be formed using digits 1 to 9 without repetition?

Use PERMUTATION P(9,4) because the ORDER matters in a number. 1234 and 4321 are different 4-digit numbers. P(9,4) = 9!/(9-4)! = 9876 = 3024. Think of it as: 4 positions to fill. Position 1 (thousands): 9 choices. Position 2 (hundreds): 8 remaining choices. Position 3 (tens): 7 choices. Position 4 (units): 6 choices. Total = 9876 = 3024. Use combination C(9,4) only when order does NOT matter, like selecting a committee of 4 from 9 people. C(9,4) = 126 represents the number of ways to choose which 4 digits to use, without regard to their order.