/* deliver mileage, given miles driven and gallons of gasoline used */
double mileage(double miles, double gallons);

/* car value after given number of miles,
   assuming an exponential model of declining value
   from a given purchase price to 5% of that price
   after 100,000 miles */
double value(double purchasePrice, double milesDriven);

/* average depreciation-cost per mile,
   assuming an exponential model of declining value
   from a given purchase price to 5% of that price
   after 100,000 miles */
double averageDepreciationPerMile(double purchasePrice,
                                  double milesDriven);