This is probably really simple but here you go.

I'm trying to convert hours worked into actual time.

If I worked 7.75 hours I want it to display 7:45 (7 hours 45 minutes).
How can I accomplish this? I would like for this to calculate since I
work by project and keep track of the minutes/hours I've spent working
on that project.

Changed Battery .25 (.25 being 15 minutes)
Changed Tire 1.75 (1.75 being 1 hour 45 minutes)
Checked Oil .25

Total: 2:15 Total Time

Any help would be great... any SIMPLE answer is greatly appreciated.