Need help with exponential question!

[codenorth]

A used-car dealer sells a five-year-old car for $4200. What was the original value of the car if the depreciation is 15% a year?

 

[codenorth]

I got 4200=No (.85)^5
so No = approx 9500
i just want to ask why cant it be 1.15 for the rate of change?

 

[richardt]

Greetings: If 4200 = A(1 + r)^5, then r = -15% = -0.15. Hence 1 + r = 0.85

Rich