Suppose that it costs $15,000 per year to obtain a four-year college education. Assume that the real interest rate is 5% and that there are 45 years beyond college graduation during which an individual will work. What after-tax returns on the investment are necessary to justify making the investment on purely monetary grounds? If the post-schooling period were changed by 5 years in either direction, how would your calculation change? What does the latter exercise show?