which expression can you use to find the number?

You had the right idea, just a little off. To figure this out, draw five lines (since there are five digits) and plug in how many available numbers you can use in each line as you work to the right. For the first line, you can use 1,3,5,7,or 9, so write 5 on the first line. For the second line, you can choose from only four remaining numbers (since you cannot repeat any of the digits and one number was used for the first line) so write a 4 on the line. Use this method to fill in the rest of the lines. When you are done you should have: 5, 4, 3, 2, 1. Now, multiply all of these numbers to find the solution to the original question. 5x4x3x2x1=120 possible combinations

No, you do permutations. I think of it as 5 blanks. For the first blank, how many choices of digits do you have? 5 choices. Then for the second blank? 4. So on and on it goes, so the expression would be 5x4x3x2x1, or 5! (five factorial) which equals 120.

Hope that helped! :)