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?

There are 5 choices for the first digit, as any of the 5 given digits can be used. After selecting the first digit, there are 4 remaining choices for the second digit, and 3 choices for the third digit. Therefore, the total number of ways to write a 3-digit positive integer using the given digits without repetition is 5 x 4 x 3 = 60 ways.