How do you get a sum of two percentages?

Answer 1

This is more complicated than it first appears. You cannot simply add percentages together - you need to know what they are percentages out of.

So if you have x% being a out of b so that x = 100*a/b

and y% being c out of d so that y = 100*c/d

Then the sum of the percentages is 100*(a+c)/(b+d).

The clearest way to see that you cannot simply add up the percentages is, if you get a 50% score in one exam and a 50% score in another, you do NOT get 100% in the two exams combined!