What do "136.5/255", "75/75", "19/20", and "30/30" mean? If they are not simply fractions, which does not seem likely, what I don't know what you mean. Further, you are confusing things by using the same letters, "a", "b", "c", and "d" for both them an their respective weights.
If you had four NUMBERS, A, B, C, and D, with "weights" 50%, 25%, 20%, 5%, respectively, then the "weighted average" would be .5A+ .25B+ .2C+ .05D.