Dirk Ward borrowed $12 000 for investment purposes on May 10 on a demand note providing for a variable rate of interest and payment of any accrued interest on December 31. He paid $300 on June 25, $150 on September 20, and $200 on November 5. How much is the accrued interest on December 31 if the rate of interest was 7.5% on May 10, 6% effective August 1, and 5% effective November 1?