The root of calculating interest income is found in the formula that is described as I equals PRT. Use a simple interest formula to calculate income with lessons from a math teacher in this free video on math calculations for daily life. Expert: Jimmy Chang Bio: Jimmy Chang has been a math teacher at St. Pete College for nearly a decade. He has a master's degree in math, and his specialties include calculus, algebra, liberal arts, math and trigonometry. Filmmaker: Christopher Rokosz