A mechanical engineering graduate who wanted to have his own business borrowed $350,000 from his father as start-up money. Because he was family, his father charged interest at only 4% per year. If the engineer was able to pay his father $15,000 in year 1, $36,700 in year 2, and amounts increasing by $21,700 each year, how many years did it take for the engineer to repay the loan?