Final answer:
The taxpayer has a $180,000 gain on the sale of the office building.
Step-by-step explanation:
To calculate the taxpayer's gain or loss on the sale of the office building, we need to determine the adjusted basis and the selling price. The adjusted basis is the original purchase price minus the depreciation taken. In this case, the adjusted basis is $200,000 - $80,000 = $120,000. The selling price is $300,000.
The gain or loss is then calculated as follows:
Gain or Loss = Selling Price - Adjusted Basis
Gain or Loss = $300,000 - $120,000 = $180,000.
Therefore, the taxpayer has a $180,000 gain on the sale of the office building.