Assuming that your fields are datetime fields, you can use the DateDiff and Average functions to help with this.
Create a formula that looks like this:
Average(DateDif('n', {table.DispatchTime}, {table.ArriveTime}))
This will give you the average number of minutes difference between the two times.
If you want to show this in hours and minutes, you'll need to create a second formula that references the first (which I'll call
{@AveMinutes}):
Notice that I used a back-slash in the first calculation ('\' instead of '/'). This is the "Integer Divide" operator that gives you the integer part of the division result.
-Dell