Averaging Time after using formula to determine time difference

I am creating a worksheet that utilizes a start time and finish time. I have been able to use =TEXT(D4-C4,"h:mm") to obtain the amount of time that has passed. Now I am trying to average that by the number of records/rows filled out (which I have totaled using =COUNTA(A4:A29)).

For example:
Column A is customer ID.
Column C is start time
Column D is finish time
Column E is Amount of time.

Further down the worksheet I have a summary section
Number of Customers: =COUNTA(A4:A29) <-- Cell B33
Average Length of Appointment: ?????

Everything I've found online is either too basic or way over the top. Help!

Thanks!

I am open to other suggestions if there is an easier way to do this.

Possibly both. I don't know how many entries will be made per day in the worksheet. Since it will be variable, I used the COUNTA formula to determine how many records/rows were used. I would need the average times based on that COUNTA number.

Here is the file with generic info.

Exams.xlsx

You're awesome! Thanks!