Suppose I have computed the cost of carbon per mile for my car at $0\mathrm{.}012$ per mile. Assume that the interest rate is $4\%$ and that I drive the car $28,000$ miles per year. What is the present value of the carbon expense for five years? You will need to use the PV function in Excel or a financial calculator. In Excel the present value of a stream of monetary cash flows is given by $-$PV$($rate$,$ number of payments, payment amount$)$ where rate is the interest rate. We enter the payment in Excel as a positive number. You can also compute the carbon costs at the end of each year and then discount those five values back to the present.