Abstract
Initial value of the sum of a series of equal payments of R dollars at equal time intervals is V and terminal value is W. It is desired to find total number (N) of such payments and rate of effective periodic compound interest (P) implied by R, V, W under a specified protocol relating start and end of a contract (e.g., a debt repayment, annuity, or lease) to times of first and last payments. Two new equations are derived, completing the set of five needed to solve for any unknown, given any three of the remaining variables and an appropriate protocol. A list of implied constraints has been provided to facilitate early detection of invalid values assigned to trios of "known" variables and to help ensure that required guesses at value of unknowns do not violate implied limits. Erroneous and correct relationships between periodic rate (P) and annual percentage rate (APR) are compared. North. J. Appl. For. 11(2):58-62.