Create total time formula

1.     I have columns of time/date values in this format:  0:02:50  (meaning 2 minutes and 50 seconds).

        I would like to add theses values up, then find the average.

        So, if I have 10 time values, I need to get the total sum of the time and then divide by 10.

        How do I create a formula in Crystal Reports to do this?

2.     The above time values are part of a Group.  I plan to hide the details and show just the Group "average" (computed in Step 1)

        How/where do I place this in my report?

Thanks for your help.