Average rating of all users

how to make a total average rating of some users who give ratings?
for example in a post there are 5 users who give a review / rating,
user 1 -> 5 rating
user 2 -> 4 rating
user 3 -> 3 rating
user 4 -> 4 rating
user 5 -> 5 rating
So, The total rating for the post is 4.2
how to make it?