Hello!
lets solve your equation step by step, and with multiple solutions.
the first way to solve it is by simply adding the hours and the minutes together, then combine them.
8+3=11
12+15=27
you get 11 hours & 27 minutes
_____________________________________________________________
for the second one, we will be using minutes only, and there are 60 minutes in an hour
the second way to solve it is:
8*60=480
3*60=180
480+12+180+15=687
I hope this helps, and have a nice day.