Converting Time into Percentages

I am creating an excel sheet to help simplify payroll. My hours and minutes is not converting to a percentage. For example, start - 7:00 am, stop - 11:30 am, 4 hrs and 30 minutes. I want this to be end up with the value of 4.5 not 4.3 (because obviously 30 minutes is half of the hour). How do I adjust this value in my excel doc?