One way to look at this situation is as follows:
Day 1 purchase of 180 units @ $5/u = $900
Day 2 purchase of 280 units @ $6/u = $1680
Day 3 purchase of 220 units @ $7/u = $1540
Summing the columns, we get 680 units with a value of $4120
At the end of the month, there were 240 units in stock, which means that
680 units - 240 units = 440 units sold
with the FIFO system, we would have used all of the inventory from Day 1 (180 units @ $5/u = $900) leaving 260 units (440 units sold - 180 units from Day 1) to be taken from Day 2 inventory.
The value of the Day 2 inventory was 260 units @ $6/u = $1560.
The total cost with the FIFO system would have been $2460.
With the LIFO system, all of the Day 3 inventory would have been used (220 units @ $7/u = $1540) leaving 220 units to be taken from Day 2 inventory.
The value of Day 2 inventory was 220 units @ $6/u = $1320, and the total cost with the LIFO system would have been $2860.
When we subtract LIFO - FIFO, we get $2860 - 2460 = $400
So, the phantom profit was $400