1.11 In return for payments of 2,400 at the end of January, March and May, and payments of 3,600 at the end of July, September and November, an investor agrees to pay the total value of the 6 payments on January 1, and to either make or receive an additional payment on December 31. Find the amount of that additional payment on the basis of a nominal annual interest rate of 7% per annum convertible monthly