How many ways are there to write a 3 digit positive integer using digits 1 3 5 7 and 9 if no digit is used more than once?

This is a question of permutations; the answer is equal to the factorial of 5 (number of digits) divided by the factorial of 3 (number used in each selection), written 5! / 3!. This equals 120 / 6, or 20 ways.