I need to know when question ask about depreciation using straight line and in question did not mention estimated useful life. How do i determined useful life and calculate? Any formula for using this method?
If it is straight-line depreciation, then you will always be told either the useful life, or alternatively be told the depreciation as a %.
As I explain in my free lectures, straight line over (for example) 4 years, is the same as depreciating at the rate of 25% on cost. Similarly, straight line over 50 years is the same as depreciating at the rate of 2% on cost.