Jayden is a songwriter who collects royalties on his songs whenever they are played in
a commercial or a movie. Jayden will earn $30 every time one of his songs is played
in a commercial and he will earn $120 every time one of his songs is played in a
movie. Jayden's songs were played on 4 more commercials than movies, and his total
earnings on the royalties from all commercials and movies was $420. Write a system
of equations that could be used to determine the number of commercials and the
number of movies on which Jayden's songs were played. Define the variables that use to write the system.