A = W 1 ∑ R 1 + W 2 ∑ R 2 + W 3 ∑ R 3 + . . . W 1 ∑ N 1 + W 2 ∑ N 2 + W 3 ∑ N 3 + . . . {\displaystyle A={{W_{1}\sum R_{1}}+{W_{2}\sum R_{2}}+{W_{3}\sum R_{3}}+... \over {W_{1}\sum N_{1}}+{W_{2}\sum N_{2}}+{W_{3}\sum N_{3}}+...}}
