has determined that city$/$highway combined gas mileage of the Green VUE and regular VUE models are $27$ and $23$ miles per gallon respectively. Mary anticipates she will travel an average of $12,000$ miles per year for the next several years.

$($Round all of your answers to two decimal places. For example, enter $8\mathrm{.}84$ for $8\mathrm{.}844$ and $8\mathrm{.}85$ for $8\mathrm{.}845.)$

$($a$)$ Determine the payback period of the incremental investment if gasoline costs $$3\mathrm{.}50$ per gallon.

years

$($b$)$ Assuming that Mary plans to keep the car five years and does not believe there will be a trade$-$in premium associated wi hybrid model, determine the net present value of the incremental investment at an eight percent time value of money. $($Use negative sign with your answer.$)$