The first thing that's easiest for me, at least, is to make the mixed number into an improper fraction. That would make this problem 5 x 15/4. Just make five into a fraction, 5/1 and then put it into the problem:
5/1 x 15/4 = 75/4
Now, put the answer you got back into a mixed number (if your teacher wants you to)
How many times does four go into 75? 18, with a remainder of 3. The answer as a mixed number would be 18 3/4 or 18.75