The impairment loss of the equipment is $-80,000.
The impairment loss of an asset is determined by subtracting the asset's carrying value from the fair value of the equipment.
Impairment loss = Fair value of the asset - carrying value of the asset.
The carrying value of an asset is the cost of an asset less the accumulated depreciation.
Carrying value = Cost of the asset - accumulated deprecation
$900,000 - $420,000 = $480,000
Impairment loss = $400,000 - $480,000 = $-80,000
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