Why is multiplying sums of numbers different from multiplying the numbers and then adding?

Question

X = population density factor Y = city population Z = city area If I group 4 cities' Xs, Ys, and Zs together and input those sums into the equation--X*Y*(Y/Z)--I get a different answer than if I calculate each city individually. Why is that and what is the term that explains it?

Answer 1

?????? multiplying???? adding ??????

wi dont yu add & multipli????????????

yu duit rong, if yu want averaej density, add up evreewon, then divide bi area