If no digit can be used more than once in a number, how many 2 digits numbers can be formed using only the digits 7, 1, 3, and 9?

Hey there!

What I would do to solve this problem is list out all four of the numbers, then place the other three numbers behind it, like so:

71
73
79
17
13
19
37
31
39
97
91
93

These are all of your possible combinations without any repeats. There's a total of 12 possible combos. 

Hope this helped you out! :-)