Answer:
 5.6 years
Step-by-step explanation:
The exponential depreciation formula for the value of a car can be solved for time by making use of logarithms.
__
We are given ...
 V = C(1 -r)^t . . . . . where C = $17,500, r = 20%, V = $5,000
and we are asked to find t.
Substituting the given values, we have ...
 5000 = 17500(1 -0.20)^t
 5000/17500 = 0.80^t . . . . . . . divide by 17500
 log(2/7) = t×log(0.80) . . . . . simplify and take logarithms
 t = log(2/7)/log(0.80) ≈ 5.614
The car is about 5.6 years old when its value is $5000.