Cecilia has just purchased a house for $200,000 with a down payment of $40,000. If she gets a 30-year fixed mortgage at 4-1/8 percent, how much is her monthly principal and interest payment? She gets a roommate and can pay an additional $500 monthly toward the principal. Assuming she either keeps the roommate or finds another way to pay the additional $500, how may months early will she pay off the mortgage?