calculiting time

yesterday, I learn about how to find the total of this question 9:15 to 12:30. first,subtract the 9 10 12 and we get 3. If you use a calculator, you will get negative 3,But we were talking about the hours, so you must put the negative sign even if you got it. with negative, because there's no negative sign when were talking about hour's. then the next thing to do subtract 15 minutes to 60 minutes(1 hour) and you got 45 minutes,then subtract again the 30 minutes to 60 minutes (1 hour) and you got 30 minutes.then subtract those minutes that you subtract before you will get 15 minutes. then the final answer is 3:15 minutes