Assume a dealer is offering a last-year passenger car model for $20,400 today or 4 year financing terms for $109 weekly payment (a total of 209 payments with the first payment made now). For the dealer, it does not make an economic difference if the customer chooses any of these two options (options are equivalent).

[a] What is the effective annual interest rate (ieff = ?) implied in this offer assuming that compounding is weekly?

[b] If the buyer is interested more in monthly payment instead of weekly, how much this monthly payment is expected to be using the same interest rate in [a] and assuming, still, that compounding is weekly?