Trying to calculate elapsed time in 2 form fields - help!

I'm attempting to calculate the total number of time in numeric expression (15 minutes expressed in ".25", 30 minutes expressed as ".5", 45 minutes expressed as ".75", you get the idea) using two fields where a user enters a start time and an end time.

Here's the calculation in simple terms:

StartTimeofCourse: 13:00pm

EndTimeofCourse: 17:30pm

TotalHoursRequesting: 4.5hrs 

I was able to get as far as a result of "4.30" by:

1. created text field

2. formatted as a number with 2 decimal places

3. calculated as a simplified field notation "EndTimeofCourse-StartTimeofCourse"

I've Google-searched the cr@p out of every text combination I can think of and just cannot find (what I think should be) a simple translation.  Before I head over to Code Academy (which I don't mind at but am on a time crunch), I'm hoping a Forum rock star can help me!