Answer:
Given
Cost $46000
Life= 5 years
Salvage Value= $ 4000
Total miles = 165,000
Formula
Depreciation Straight Line Method= Cost - Salvage Value/ Useful Life
Straight Line Rate= 100%/ useful Life= 100%/5 = 20%
Double Declining Method = 2 * Straight Line Rate
Double Declining Method = 2 * Straight Line Rate= 2*20%= 40%
1.
Depreciation Straight Line Method= Cost - Salvage Value/ Useful Life
Depreciation Straight Line Method= $ 46000- $4000/ 5= $ 8,400
The depreciation expense using the straight line method does not change unless the salvage value is reached
Years        Depreciation      Accumulated Dep          Book Value
                                                                                 (Cost - Accu. Dep)
a. 2021       $ 8,4000               8400                            37600
b. 2022       $ 8,4000               16,800                         29,200
c. 2023        $ 8,4000              25200                          20,800   
d. 2024        $ 8,4000              33,600                        12,400
e. 2025        $ 8,4000             42000                        4000
2. Straight Line Rate= 100%/ useful Life= 100%/5 = 20%
Double Declining Method = 2 * Straight Line Rate
Double Declining Method = 2 * Straight Line Rate= 2*20%= 40%
In double declining method the rate is multiplied to the cost to get the depreciation expense. 40 % of $ 46000= $ 18400
Each year the rate is multiplied with the remaining book value after deducting the depreciation expense from the cost as $ 46000- $ 18400= $ 27600
Next years depreciation will be $ 27600 * 40%= $ 11040.
This will be added in the original depreciation expense $ 18400 + $ 11040 = $ 29440 and deducted from cost to get the book value. $ 46,000- $ 29440 = $ 16560.
Again rate will be multiplied and each years depreciation will be calculated similarly.
It has been summarized in the table below.
Years       Dep Rate      Dep Expense       Accu. Dep.     Book Value
a. 2021        40%            18400                    18400               27600
b. 2022       40%            11040                     29440               16560 
c. 2023       40%             6624                     36064               9936
d. 2024       40%             3974.4                   40,038.4         5961.6
e. 2025       40%            2384.64                   42,0423.4     3576.96
3. Depreciation per unit= (Cost -Salvage value) / Total units of production* Units of Production
Years       Mileage      Depreciation                    Depreciation
a. 2021      35,000     ($ 42000/165000)*35000        8909.09
b. 2022     37,000      ($ 42000/165000)*37000       9418.18
c. 2023      28,000     ($ 42000/165000)*28000        7127.27
d. 2024      33,000      ($ 42000/165000)*33000        8400
e. 2025      34,000    ($ 42000/165000)*34000         8654.54