running total and average of that total after 3 events
 

I have a sheet that I use as a from 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 ben on the
press after 3 consecutive times of being chnaged 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 he 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 ma not sure if whatI want to do is possible but I know there are alot of
creative thinkers here

Thanks for the help

DN
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crunchin numbers