I think the relationship between the weight of a stack of standard 8.5x11 copier paper vs number of sheets of paper is proportional because :
Let us assume here, in this situation that each page has the same weight so if i want to find the weight of the complete stack, i will multiply the number of sheets of paper by the weight of a single sheet of paper.
First let's get rid of the in the denominator of the fraction on the right side of the equation. This will help to get only and a coefficient on one side of the equation: