The ordinary annuity present value factor converts a level payment stream into today’s value when payments occur at the end of each period. In words, it is the sum of discounted ones for N periods. Lightly: A = [1 − (1 + r)^(−N)] / r. Multiply A by the payment size to get PV. Using beginning-of-period (annuity due) timing when the problem is ordinary is a classic trap.