calculating times in 15 minute increments

Good Afternoon,

I am making a form where we report how much leave is being used. The leave will be reported in 15 minute increments. (Ex: fifteen minutes = .15; one-half hour = .30; 45 minutes = .45) I need to be able to add these up and have them keep this format of 15 minute increments. Also, I need the display to be a decimal instead of a colon, if possible. (Ex. 3.45 instead of 3:45).

any advice is appreciated!!!

Gene-O