title: 1 raiting subtraction from total weighted rating
body: <p>This is a 10 star rating.</p>
  
  <p>I have a weighted rating of 8.1 made from 25 users ratings.
  I want to delete 1 user rating (7.5) that is invalid, how i do direct calculation on this? I don't want to recalculate all 24 ratings that are left.</p>
  
  <p>I tough of this (8.1+7.5)/2 but it doesn't give me the right answer</p>
  
  <p>Any others ideas of subtraction from a weighted rating?</p>