Time Value of Money.See Corresponding paper under the Papers link in the left sidebar.
Notice: Negative value for owed balance and money paid to the account, positive values for balances in your favor and withdrawn from the account.
Basic financial formulars:
CF=Compound frequency, PF=payment frequency, X=1 for beginning of period payment otherwise 0, i=effective interest
Financial equation: (General)
which solved for the other variable gives:
(9): IR solved by iteration