Decision variables: There should be $9$ main decision variables: the number of each car type, the

amount of the down payment, the amount of payment in Year $1$ and Year $2,$ and the amount of

interest in Year $1$ and Year $2.$ You can define additional definitional decision variables to make the

model easier to read.

$$ Net present values: the NPV of the two$-$year revenue streams for each vehicle type is obtained by

discounting the first$-$year revenues by the discount rate $=10\%$ and the second$-$year revenues

twice by $10\%.$ These are the coefficients required to set up the objective function.