GRE Strategy for Quantitative Comparison
GRE quantitative comparison problems consist of two columns labeled A and B. Each column contains a number or an expression. You have to decide whether A or B is greater, if they are equal, or if the relationship between A and B cannot be determined from the provided information. Your answer choices are:
A. The quantity in column A is greater.
B. The quantity in column B is greater.
C. The two quantities are equal.
D. The relationship cannot be determined from the information given.
To find the answer to a QC problem, you can calculate the quantities in both columns, or you can plug in numbers (which is often easier and faster).
Plugging in Numbers
Let's take a look at an example to strategize how we plug in numbers.
| Column A | Column B |
| x2 | x4 |
Say we plug in x=2. Then column A equals 4 and column B equals 16. So our answer is B, since the quantity in B is greater, right? NO! We only tried one number. What if x=1? Then both columns would equal 1 (choice C)! What if x=1/2? Column A would equal 1/4 and column B would equal 1/16. Column A is would be greater. Since we cannot determine which column is greater, our answer is D.
From this example we learn that we should plug in different types of numbers. In general, you should try small and large negative integers, small and large positive integers, positive and negative fractions, zero, and one.
If you come across a single discrepancy when you are plugging in numbers, then you can choose D. For instance, in our example, the moment we found that the answer choice could be B when x=2 AND C when x=1, we could have answered D. We didn't have to plug in more numbers. We only continued for demonstration sake.
Bottom Line
When you are doing GRE Quantitative Comparison problems, plug in a wide spectrum of numbers. Practice this method to hone your skills.