From the given original cost and the total accumulated depreciation of the old equipment, it can be seen that the value of the equipment should still be $180,000.
salvage value = $600,000 - $420,000 = $180,000
Given that it can be sold for only $18,000, the sunken cost is $162,000.
sunken cost = $180,000 - $18,000 = $162,000