I am unsure how to calculate this problem. What formula should i use?

In its first month of operation, Windsor, Inc. purchased 320 units of inventory for $4, then 420 units for $5, and finally 360 units for $6. At the end of the month, 400 units remained. Compute the amount of phantom profit that would result if the company used FIFO rather than LIFO. Phantom profit $ Save for Later Attempts: 0 of 1 used Submit