So i have a formula that I need help with - big surprise lol!

Let's say I have the following info:

10
5
5
50
400

I need to get the average per sale from the 400 coming from the 50 total
sales - but need the two 5 sales out, although they need to remain in the
tracker! I'm sure it's possible, but if I can get some assistance that would
be fantastic!

Thanks