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?