A movie streaming website charges an initial fee of $50, plus $5 for each movie that is streamed. How many movies must be streamed for the average cost per movie to fall to $7?

The average cost of streaming N movies will be

... average cost = (total cost) / (number of movies)

... = (50 + 5N) / N

... = (50/N) + 5

You want this value to be 7, so you have

... 7 = (50/N) + 5

... 2 = 50/N

... N = 50/2 = 25

25 movies must be streamed to get the average cost down to $7.