The yearly depreciation rate for a certain vehicle is modeled by $r=1-\left(\frac{V}{C}\right)^{1 / n}$, where $V$ is the value of the car after $n$ years, and $C$ is the original cost.
a. Determine the depreciation rate for a car that originally cost $\$ 18,000$ and is worth $\$ 12,000$ after 3 yr. Round to the nearest tenth of a percent.
b. Determine the original cost of a truck that has a yearly depreciation rate of $15 \%$ and is worth $\$ 11,000$ after 5 yr. Round to the nearest $\$ 100$.