In a store, you are offered to sell a television set for $ 700, including sales tax. The latter is $ 57.80 and you have to pay it now, in cash, that is, it is included in the price of $ 700. The remaining amount, $ 642.20, should be paid in 12 equal installments of $ 70 each month-end, with the first payment starting in a month.

The merchant charges you a periodic (monthly) rate of: