The rule of 78s is a commonly used method of computing the amount of interest when the balance of a 1-year loan is repaid in advance. Adding the numbers from month 1 to month 12 equals 78. Now the first month's interest is 12/78 of the year's interest. The second month's interest is 11/78 of the year's interest. Thus after 11 months the total interest charged would be 77/78 of the total year's interest. Helen borrowed $10,000 at 15% annual interest, compounded monthly. The loan was to be repaid in 12 equal end-of-period payments. After making the first two payments she decided to pay off the balance along with the third payment. Calculate the amount of this additional sum (a) based on the rule of 78s (b) based on exact economic analysis methods. (c) How close is the approximation