I need to calculate the elapsed time in hours from data in 4 date and time
columns (date dd/mm/yyyy format and time in 24 hour format) e.g.

Start Date Start Time Stop Date Stop Time
10/12/2008 15:10 13/12/2008 12:50
11/12/2008 02:00 11/12/2008 16:30
11/12/2008 10:10 13/12/2008 09:00

I need to then be able to find the average elapsed time in hours.

Manually the solution for the above is:

69:40
14:30
46:50
Average elapsed time: 43:40

As I am not used to working with date and time fields, any help would be
much appreciated as I have several hundred lines of data and manually
calculating this would drive me nuts.

~John