112 - 40 = 72. 112 represents the total time, but you are comparing music to commercials, so you have to subtract the commercial time from the total time.
The correct answer is: if is a root of , then is also a root of .
In fact, every polynomial has real and/or complex solutions. If all solutions are real, we're good. But if not all of them are real, then the complex ones come in couple of conjugate solutions. Since and are conjugate complex numbers, if one of them is a solution, the other must be as well.