There are calculators for that but the easy way is to take a common mortgage calculator and punch in the figures you gave. This comes out to $620.24 per month. After 10 years (120 payments) you'll have a balance of $53,418.78. That'll be your balloon. There's no additional interest as each monthly payment between now and then is the interest on the entire amount owed (on that month) with the amount of your payment over that going towards principal. Hopefully this makes sense.

You can do other examples at

http://easyrentaltools.com/tutorials/mortgage-calculator-1.cfm

After putting in the numbers (80,000 , 7%, 20 years) hit submit. The results will show a "show payment schedule" button. Hit that and scroll down to payment 120 (10 years). It'll show the balance due at that point (or any other) which will be your balloon at that point.

Hope this helps.

You can do other examples at

http://easyrentaltools.com/tutorials/mortgage-calculator-1.cfm

After putting in the numbers (80,000 , 7%, 20 years) hit submit. The results will show a "show payment schedule" button. Hit that and scroll down to payment 120 (10 years). It'll show the balance due at that point (or any other) which will be your balloon at that point.

Hope this helps.