Create a loan amortization chart on excel and answer the following question

A couple bought a $$100,000$ house six years ago using $10\%$ down and a $15$ year mortgage with

an annual interest rate of $7\mathrm{.}8\%.$ What are their monthly payments? What percentage of their total

payments over the last six years went to paying off the principal? Starting today the couple

decides to pay $$100$ extra on their monthly payment, how long will it take to pay off the house?