Amos takes out a 25-year repayment (amortising) mortgage of 100,000 at an annual interest rate of 6% to buy a home. He does not want to have to pay the mortgage for 25 years and intends to pay the mortgage off early by simultaneously investing 9,000 in an investment product that pays a compound annual rate of interest of 4%. At the end of which year will Amos be able to pay off all the outstanding mortgage?