The Continental Bank made a loan of $20 000 on March 25 to Dr. Hirsch to purchase equipment for her office. The loan was secured by a demand loan subject to a variable rate of interest that was 7% on March 25. The rate of interest was raised to 8.5% effective July 1 and to 9.5% effective September 1. Dr. Hirsch made partial payments on the loan as follows: $600 on May 15; $800 on June 30; and $400 on October 10. The terms of the note require payment of any accrued interest on October 31. How much must Dr. Hirsch pay on October 31?