Problem 5-3A Perpetual: Alternative cost flows LO P3

I am not able to understand how to set FIFO and the other part of the question the financial accounting book explain something and the question is different as below

Problem 5-3A Perpetual: Alternative cost flows LO P3 Montoure Company uses a perpetual inventory system. It entered into the following calendar-year 201 purchases and sales transactions Activities Units Sold at Retail Date Jan. 1 Beginning inventory 600 units Feb. 10 Purchase Mar. 13 Purchase Mar. 15 Sales Aug. 21 Purchase Sept 5 Purchase Sept. 10 Sales Units Acquired at Cost S45 per unit 400 units @$42 per unit 200units $27 per unit 800 units $75 per unit 100 units @ $50 per unit 500 units $46 per unit 600 units @ $75 per unit Totals 1,800 units 1,400 units Required: 1. Compute cost of goods available for sale and the number of units available for sale Cost of goods available for sale 77,200 Number of units available for sale 1,800 units 2. Compute the number of units in ending inventory Ending inve 400 units