Let x stand for a movie.
You can pay $4 every movie or $1.50 for every movie plus the original $10
4x = 1.50x +10
-1.50x -1.50x (Subtraction Property of Equality)
2.50x = 10
/2.50 /2.50 (Division Property of Equality)
x = 4
After 4 movies, the membership will cost less.