Example $5\mathrm{.}5$ You know that you need to buy $$400$ in textbooks in one year. You can earn $7\%$ on an investment for one year. How much do you need to put up today to have enough for your textbooks?

Example $5\mathrm{.}6$ You have $$50,000,$ but the car you want costs $$68,500.$ If you can earn $9\%,$ how much do you have to invest today to buy the car in $2$ years $($assume price won't change$)?$ Do have enough?