Joan borrowed $15 000 to buy a car. She repaid $2000 two months later and $5000 seven months later. After 12 months, she borrowed an additional $4000, and repaid $3000 after 16 months. She paid the entire balance, including the interest, after 24 months. Interest was 7% compounded monthly for the first year and 7.5% compounded monthly for the remaining time. What was the size of the final payment?