I am trying to find a formula that will do the following.

Say I am buying and selling stocks.

On day 1, I buy 5 shares of ABC stock at $10 each.
On day 2, I buy 10 shares of ABC stock at $15 each.
On day 3, I buy 15 shares of ABC stock at $20 each.

On day 4, I sell 20 shares of ABC stock at $18 each.

What I need is a formula that will take the amount I sold (20 shares),
subtract out the first 5 shares I bought on day 1, then subtract the 10
shares I bought on day 2, and then only take 5 of the share bought on day 3
because that is all that is left.

I need this as automated as possible, but everytime I think I have it
figured out, the numbers don't work.

Any help would be more than appreciated!