pls lydia is a songwriter who collects royalties on her songs whenever they are played in a commercial or a movie. lydia will earn $20 every time one of her songs is played in a commercial and she will earn $70 every time one of her songs is played in a movie. lydia earned a total of $510 in royalties on 13 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 lydia's songs were played. define the variables that you use to write the system.