Hi there, kim-z. 

QuickBooks would first base on when the item was purchased. Since you recorded a bill and the items on the same date, QuickBooks will get the most items available. In your case, QuickBooks can accommodate Qty= 15 on the Rate=6. 

Here's an article that will guide you what FIFO is and how is it used for inventory cost accounting. 

Get back to me if you have further concerns.