Mason is a songwriter who collects royalties on his songs whenever they are played in a commercial or a movie. Mason will earn $30 every time one of his songs is played in a commercial and he will earn $80 every time one of his songs is played in a movie. Mason earned a total of $470 in royalties on 9 commercials and movies. Write a system of equations that could be used to determine the number of commercials and the number of movies on which Mason's songs were played. Define the variables that you use to write the system of equations.