Mr. Robie, owner of the Black Cat Bakery owes $21,600 on a 4.5%, 90 day note. On Day 10, he makes a partial payment of $1,600. On Day 30, he makes an additional partial payment of $8,000. On Day 60 he makes yet another partial payment of $750. Assuming ordinary interest, what is the adjusted balance due at the end of the 90 days? Remember to use the U.S. Rule! (Round to the nearest cent)