New underwater camera and sonar accessories are going to be installed on the fishing boat to detect the movement of mackerel fish up to 133 ft water depth. It is estimated that the present worth now of income from the new installation is $13,500,000. There will be no income in years 1 to 3, but in year 4 income will be $200,000, and the amount is arithmetically increased through year 15. If the interest rate is 12% per year, find the required gradient.