Weighted average of ever changing stock

I buy 5000 cases at $10 and than buy 3000 cases at $15. I know how to calculated weighted average of this ($11,875).
But now I sell 6000 cases so based on the fifo concept i have now a stock of 2000 cases at a weighted value of $15 because all 5000 cases at $10 are already sold. So, how do I calculate the weighted average of my actual stock not considering the prices of cases that are no longer in my stock.