Nora is a songwriter who collects royalties on her songs whenever they are played in a commercial or a movie. nora will earn $40 every time one of her songs is played in a commercial and she will earn $120 every time one of her songs is played in a movie. nora earned a total of $640 in royalties on 12 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 nora's songs were played. define the variables that you use to write the system.