I think it would by understood better if you say its "weighting is 20%", instead of "weight is 20%". This sounds like a question in statistics. Though I am not well-educated in statistics, I have heard the term "weighted score". I think that is the term you would want to use.

Do I understand this right? Maybe you have four units in a course, or four parts in an exam, and the "weights" of each unit are: 10%, 20%, 30%, 40%. So if you score 90 out of 100 in the section that "weighs" 20%, then yes, that converts to 18 points in the overall score. Hurrah for maths! But to say it... hmmm, I'd just explain it: "I scored 90 in the part weighing 20%. That's 18 points towards my total mark." Just using "weight score" is confusing.

What do you mean by "item's score"? And 20% of what? Your question doesn't make sense.

weight and yours. you say what you want!