GRE Quantitative Practice, Quantitative Comparison
Question 2
3x – 10 = y 3y – 10 = x  
Quantity A x  Quantity B y 

Explanatory Answer
At first glance the centered information might appear to provide the same equation but in two different forms. If this were the case, the correct response would be choice (D), since you cannot solve a system of two equations in two variables. But the two equations are not the same. To make the comparison, apply algebraic substitution. For example, you can substitute 3y – 10 for x in the first equation to solve for y, and then plug that value into either equation to find x. You'll discover that x and y both equal 5. The correct response is (C).