If this can be modelled by exponential decay, therefore the function must be in the form of:
A = Ao (1 + r)^t
where A is the final value, Ao is the initial = 15000, r is the rate of decay = -0.0015, t is time = 2010 – 1990 = 20
A = 15,000 (1 – 0.0015)^20
A = $14,556.36