The standard equation for depreciation is
FV=PV(1-i)n
where
FV=future value
PV=present value
i=depreciation rate per annum, in decimals
n=number of years
Example:
Find the value of a car after 10 years if it was purchased for 50000 and depreciated at 20% per year.
PV=50000
i=0.20
n=10
FV(after 10 years)
=PV(1-i)^n
=50000(1-0.20)^10
=$5368.71