Depreciation allocates an asset's depreciable cost over the periods that use it. Straight-line depreciation subtracts estimated salvage value from cost, then divides the remainder by useful life. It does not estimate resale price each year and it does not create cash for replacement. Credit Accumulated Depreciation so original asset cost stays visible.
For partial years, follow the stated convention consistently. These exercises start depreciation in the placed-in-service month and include a full month for that month. Purchase date and in-service date may differ. The final period uses only the amount needed to reach salvage. Land is not depreciated. Changes in estimates apply prospectively; do not rewrite prior years merely because a later estimate changes.