TITLE: How to properly find the average percentage of a sum of percentages
QUESTION [2 upvotes]: I’m working with a feature that was broken in several parts, and now my manager wants to know how much is left in each piece and the average percentage as well.
The percentages are 80% + 80% + 50% + 25%
From what I understand I should sum everything and divide by 4, which gives me 58,75%. I would like to know if I’m correct in this assumption or not.

REPLY [1 votes]: The answer to your question 

I would like to know if I’m correct in this assumption or not.

is maybe, but probably not.
Whether it's correct depends on how the original feature was broken into parts. Suppose there were $100$ subfeatures and the four parts consisted of $40, 40, 8$ and $12$ subfeatures each. Then you would have have left $32 + 32 + 4 + 3 = 71$, so $71\%$ of the original task.
In your example the simple average would be correct if you knew that the four parts were the same size.
You cannot simply average percentages. You need a weighted average where the weights determine how much each of the original percentages contributes to the total.
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