How do they represent the average movie's rating on imdb, rotten tomatoes?

Alright lets say the average rating of a movie/restaurant is 4.3. On the show page you have the name of the object, the description, the contact, images etc etc... then you have the average rating represented in stars. In the object's model you store the rating from every user and with simple mathematical formula you calculate the average. But what I am curious about is how do they represent 4.3 which is 4 stars and a little bit less than 4.5? Isn't it too much to have stored 50 png images? Thanks in advance.

via G. Nick