Need to determine the # of hours between a start and end time AND be able to add them up...

I am trying to find out how many hours certain rooms are used over a period of time.  I can accomplish part of this problem, but can't figure out how to get the final result.  I have a start time and an end time.  I can come up with the number of hours the room was used - such as 2 1/2 hours or 5 hours, etc.  But the format for that calculate is a time format.  When I try to add up the number of hours (2 1/2 hours + 5 hours should be 7 1/2 hours) it doesn't work.  The result is some odd number.  

I've tried to copy just the value of that cell but instead of 2 hours it comes up with something like .853245.  How can I  calculate the number of hours used so that I can add up the totals and get the right number of hours?