We have to set our variables before we start solving. After setting the variables, we can set up the equations. If x = # of commercials and y = # of movies, then we can assume x + y = 90, as there have been a total of 90 commercials and movies played altogether. Then, we know that their worth is $500, so the price of all the commercials played and the price of all the movies played (which is 40x + 110y) sum to $500.
(I've just answered this question from your previous post. Unless my answer was wrong, I don't see why you would repost the exact same question.)
