"how is depreciation accounted for if disposal of a plant asset occurs during the year? "

When accounting for depreciation for a disposal of a plant asset when it occurs during the year, you need to record the value that was depreciated up until the point of sale. 

When an item depreciates, it means the value lost over time. When you sell an item midway through the year, you won't have the depreciation loss that happens while you own it the entire year since you sold it. Instead, we record just the amount of money that was lost when the asset depreciated while it was owned. 

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