Do you start by putting it in order?
Example dec.15,15@&25 Dec.14 8@ 25 etc

FIFO:

It assumes that the goods your purchase first, you sell first.

So, in your example, you made your purchases as follows:

Dec 14. 8 units @ $25 each
Dec 15. 15 units @ $25 each

Let's say on Dec. 17 you sell 20 units.

You will record it as selling all 8 units from Dec. 14 and 12 units from Dec. 15 purchase.

In this case, the purchase price is the same. Where you get different numbers is when the price you paid on Dec. 14 is different than the price you paid on Dec. 15