Question

How many ways can you arrange the numbers 1,3,5,7 to make a four digit number that uses each number exactly once

Answer 1

Since this isn't a more complicated probability problem, I'll try to explain. You have 4 numbers and four placements in the order of the new four-digit number. In this case, you'd take the four numbers and multiply it by the four placements to get 16 different variations.