The value of Tonya's car is $21,000. The car's value depreciates at a rate of 15% per year.

Which function represents the value of the car after t years?

f(t)=0.85(21,000)t
f(t)=1.15(21,000)t
f(t)=21,000(0.85)t
f(t)=21,000(1.15)t