Calculate the total amount of units sold!?

This is the example given. Oct 1 Beginning inventory.. 6 units at $170 Oct 15 Purchase.......... 7 units at $170 Oct 26 Purchase.......... 13 units at $180 Beginning Units + Units Purchased - Units in Ending Inventory= Units Sold (6) + (20) - (11) = (15) I DON'T UNDERSTAND HOW THEY GET (11) FOR ENDING INVENTORY! please explain.