Why these two average results a diferentes values?

Question

(12 + 18 + 5) / 3 (elements) = 11,67

and

(12 + 18) / 2 (elements) = 15

5 / 1 (element) = 5

(15 + 5) / 2 = 10

which one is more accurate? 11,67 or 10 ?

Answer 1

The first time, each element is given a weight of $1$, whereas the second time, you consider the average of an element with the average of the two others: this is not at all the same thing, as the results indicate clearly.

Now, there is not a "more accurate" value - they are both accurate, but correspond to different quantities. If you want an average of $3$ elements with an equal weight, then your first formula is correct.