Answer:
Double Declining Depreciation rate = 1/5*2
Double Declining Depreciation rate = 40%
Double Declining Depreciation for 2021 = $75,000*40%
Double Declining Depreciation for 2021 = $30,000
Book value at December 31, 2021 = $75,000 - $30,000
Book value at December 31, 2021 = $45,000