Depreciation could be determined in different ways. For straight-line depreciation method, the formula is: Depreciation = (Cost -Salvage)/Life expectancy
This is called straight-line because the depreciation varies UNIFORMLY with time. If you graph the depreciation with time, it will form a straight line.
Depreciation= (120,000-90,000)/3 years
Depreciation = $10,000 per year