To find the depreciation expense using the double declining balance method, we first divide 100 by the four year useful life to get a percentage of 25%. Double that gives us 50%. The nearest whole month is October, so for 2016 we only have three months to depreciate the asset. One quarter of 50% is 12.5%. 12.5% of our $82,000 purchase price is $10,250. This is our depreciation expense, and the Net Book Value would be $71,750 for the end of 2016. For 2017 and 2018, you would use the full 50% for depreciation expense. This is subtracted from the previous year's NBV. For 2019, subtract the residual value from the remaining book value to get the final year's depreciation.