You take out a 30-year $100,000 mortgage loan with an APR of 6% and monthly payments. In 12 years you decide to sell your house and pay off the mortgage. What is the principal balance on the loan?

First find the monthly payment: Plugging in a calculator n = 30x12 or 360, APR = 6%/12 = 0.5%, present value = 100,000, future value equals 0, solving for payment gives a monthly payment of 599.55.

Next, find the what the principal is worth in 12 years: Plugging into a calculator n = 12*12 or 144, APR = 0.5%, present value = 100,000, payment = - 599.55, and solving for future value gives a remaining principal of 79,079.55.