Explanation

This is a great first Problem Solving question, because, behind the algebra, we are dealing with a ratio, and ratios are one of the least-appreciated topic areas on the Quantitative section on the GMAT.

The key to ratios is to work with fractions and take careful note of units. We can do this, starting from the end of the question prompt. We are asked for pieces of paper. The proceeding sentence tells us that we have 10 pieces of paper per folder. "Per" can be thought of as a word that indicates the line of a fraction, so we can write:



Similarly, we have . We can multiply these fractions so that the units cancel.



Notice that if you write the fractions with the units included, it's much easier to make sure you don't have the fractions flipped from what they should be. Lastly, we multiply by the number of passengers:



We're done, right? In this last step also, we can use units and watch them cancel on the top and bottom of the fraction to ensure that we are multiplying the right things. This method of using units is called (or is related to what is called) dimensional analysis by some scientists. It works well to stay organized and avoid error on both easier and more difficult GMAT questions involving ratios and/or different types of units.

As a final step, we can make sure that all the units are accounted for. And, as a matter of fact, there is a unit that's not apparent in our scratch work above: buses. We're not looking for papers per bus; we're looking for paper per two buses. So, for two buses, we have to multiply 10xy by 2 to obtain 20xy. Writing and thinking in units helps to avoid overlooking this step. The correct answer is (C).