I'd like to set a worksheet to work out the costa for goods sold of numbers
of goods which are with two different costs.

For example, Item A, 100 pcs are with cost of $100 and 50 pcs with cost of
$120. In January, 85 pcs Item A sold, the cost of goods sold is expected -
85*$100;
In February, 35 pcs Item A sold, the cost of goods sold is expected -
15*$100+20*$120. There are numbers of goods like this.

How can I set a worksheet with the formula working out the costs of goods
sold?