Sherry and Sam want to purchase a condo at the coast. They will spend $$$$620$ comma $000$ on the condo and are taking out a loan for the condo for twenty years at $7\mathrm{.}2\%$ interest.

a$.$What is the monthly payment on the$$ mortgage? Construct the amortization of the loan for the twenty years in a spreadsheet to show the interest$$ cost$,$ the principal$$ reduction, and the ending balance each month.

b$.$Then change the amortization to reflect that after ten$$ years, Sherry and Sam will increase their monthly payment to $ $7$ comma $200$ per month. When will they fully repay the mortgage with this increased payment if they apply all the extra dollars above the original payment to the$$ principal?