Your brother-in-law needs a loan again. This time he needs to borrow $20,000. He always comes to you because you only charge him 4% interest per year. He promises to pay you $4000 a year from now, $8000 in two years and the rest of what he owes you in three years. How much will his third payment be? Suppose at the end of year three he says,"Well,I've paid you $12000,what's left must be $8000." Explain to him intuitively why that's not, in fact, what he owes you.