Averaging a running total
 

I have a sheet that I use as a form to keep track of how long a part on our
press has been in service.
I want to be able to get an average of how long this part has been on the
press after 3 consecutive times of being changed out.
I am using this formula =IF(B8="","",TODAY()-B8)
this gives me a running total of how long the current part has been on the
press.
What I need is a formula that will be able to tell when I enter a new date
in B8 and take the total days from the old part and keep a runnng total until
the 3rd change out occurs and then give an average of how many days the 3
parts were on.
So if say part 1 was on 30 days and part 2 was on 25 days and part 3 was on
40 days the average would be 41.66 days
I am not sure if what I want to do is possible but I know there are alot of
creative thinkers here. If I need to Use VBA that is fine but I need help
writing the script also.

Thanks for the help

DN

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cruchnin numbers