If you want it in hours, you want to convert the 15 minutes into terms of hours. A trick is to find a conversion factor and multiply it with the 15 minutes. For example, you know that there are 60 minutes in one hour. This is your conversion factor: 60 min/hr. Now multiply it with 15 minutes such that the units cancel:
Note how the "min" cancels.
This works for anything else: