GRE Sample Test 1 Quantitative Reasoning 2

There are a lot of variables in this problem, so start thinking about Plugging In. The variable a has to be the same for each equation.
You cannot pick just any number, however, because you must satisfy the equations.
When you feel stuck on a problem, start looking at the numbers; remember the math will always work out nicely. Examining the two equations, you realize that 158 × 4 = 632, so
these two numbers are related. So the easiest number to Plug In for a is 632. Now you know that xs = 1, and ys = 4. Since the variable s is the same in both equations, they cancel each other out and you are left with x = 1 and y = 4. Eliminate (A) and (C).
Next, try a FROZEN number such as a = −632. In this case, xs = −1 and ys = −4 or x = −1 and y = −4. Eliminate (B). The correct answer is (D).