Question

How many three digit numbers can be made from the digits 1,\ldots,9 if repetitions of digits are not allowed?

Answer 1

9 values for each digit, order matters: 123 is different than 321, and no repetion

9*8*7 = 504

Look at a simpler example, with 1, 2, 3:

123
132
213
231
312
321

6 numbers

3*2*1 = 6