how do I get this answer?

Grays Company has the following purchases and sales during the month of August. Using the FIFO perpetua/inventory method, what amount will be reported as cost of goods sold for the 12 units that were sold? Units Sold at Retail Date August 1 August 3 August 6 Activities Beginning inventory Purchase Sales Units Acquired at Cost 10 units @ $26 = $260 20 units @ $28 = $ 560 12 units sold