You can write the combination formula which is in the picture. Now if you can't guess what the varibles mean I'll tell you. N is the amount of digits you're allowed to use, and k is the amount of spots allowed in a number. Plus if the factorial sign or ! isn't part of your common knowledge, it means to multiply all below it stopping at 1. Finally if you comput your information into the formula you find out that there's 24 combinations. I hope this answers your question.
