How to calculate the weighted average for a cyclic series?

Question

Mathematics

There is an array of values ​​. The first value ranges from 0 to 20 inclusive, the second - from 0 to 4000, is the "weight".
You need to find the weighted average, provided that the first number is cyclic, that is, after 20 comes 0 (for example, the average value without weights for the numbers 18 and 3 will be 0).
Help with formula please.

Answer 1

(0*3600+5*600+20*1400)/(3600+600+1400)