On January 31st, Mr. & Mrs. Gruber borrowed $500,000 from BMO to buy a house in Blainville, Quebec. The mortgage rate is quoted at 4% APR (with semi-annual compounding). The loan is to be repaid in equal monthly payments over 15 years. The first payment is due on February 29th. In their 80th payment, $________ will be used to pay back the principal? (Assume that each month is equal to 1/12 of a year).