Paris company buys a plot of land for $100,000 paying $20,000 cash and signing a 20 year mortgage note for $80,000 at 6%. Monthly payments are $570. The first monthly payment was made on January, 2013. After the first payment, what is it updated principal balance?

a.) $79,430

b.) $79,600

c.) $79,830

d.) $79,440

how do you know this?