GRE Exam: A Simple Guide To Two Essential Quantitative Reasoning Strategies

The following strategies are meant to apply generally to all content areas. Though no GRE question is “designed” for you to use these strategies, they can be effective in situations where no algebraic or conceptual solution is immediately apparent. When working through the questions, focus on mastering all approaches toward the question—both the algebraic, content-based approach and the “strategic” approach, where possible.

Strategy 1: Plug In Numbers
One of the primary difficulties test-takers have with algebra is its abstract nature. Plugging in numbers helps you get past the abstract by using concrete values in place of variables. You can plug in numbers any time there are variables in the answer choices. This does not mean that you always should plug in numbers in these situations, but it is always an option. Since plugging in numbers requires variables in the answer choices, it can only be used on multiple-choice Discrete Quantitative questions. Let’s look at a sample question and how to answer it by plugging in numbers:

Example: Which of the following equals the average (arithmetic mean) of (a − 2)2 and (a + 2)2?
a2
a2 + 2
a2 + 4
a2 + 2x
a2 + 8x

STEP 1: Choose a value for the variable in the question. When choosing a value for a variable, keep in mind the following recommendations:
- Never choose 0 or 1.
- Never choose a value that will yield a 0 or 1.
- Avoid repeating the same value throughout the question.
- Always work with integers.

So what value should you choose for a? Based on number 1, you won’t choose 0 or 1. Based on number 2, you won’t choose 3. Why? Because if you plug in 3 for a, (3 − 2)2 = 12, so 3 yields a value of 1. Based on number 3, you won’t choose 4: (4 – 2)2 = 22. 2 will appear multiple times in the question. So you can’t choose 1, 2, 3, or 4. Go with 5. It does not violate any of the rules given earlier, and it is a prime number, which you generally want. Once you have chosen the value, label it on your paper: a = 5.

STEP 2: Answer the question using the value that you chose.

Since you let a = 5, the question now becomes: Which of the following equals the average (arithmetic mean) of (5 − 2)2 and (5 + 2)2? Solve:

The average of 9 and 49 is:

The value above is called the “goal.”

Write that answer on your paper and circle it.


STEP 3: Plug the value you chose for the variable into the choices. The choice that yields a value that matches the goal will be the correct answer. This step is straightforward, except for the following caveat: Check all the choices! Occasionally, more than one choice will yield the correct answer, in which case you will need to choose a new value for the variable. This is obviously not an ideal situation, which is why you want to follow the rules in Step 1 about which numbers to avoid.

Here’s the question again:
Which of the following equals the average (arithmetic mean) of (a − 2)2 and (a + 2)2?
a2
a2 + 2
a2 + 4
a2 + 2x
a2 + 8x

SOLUTION: Plug in 5 for a, and identify which choice yields a value of 29.

The only choice that yields a value that matches the goal is C, so C is the correct answer.

Plugging In Numbers with Multiple Variables
Sometimes, you will have a candidate for plugging in numbers, but there will be more than one variable in the question.

Look at the following example:
If a + b = 11c, what is the average of a, b, and c, in terms of c?
3c
3c + 1
4c
5c
5c − 1

When there are multiple variables in a plug-in question, you must choose values that satisfy the restrictions in the question.

For example, in the previous question, you cannot simply choose 2 for a, 3 for b, and 5 for c. When you plug those values into the equation, you will arrive at 2 + 3 = 11(5) = 55. This is not a true equation. Instead of arbitrarily choosing values, you must let the values for one or more of the variables determine the other variable. In the previous example, let’s choose values for a and b and let those values determine c.

STEP 1: Choose values for the variables in the question.

What values should you choose for a and b? You can choose anything that does not violate the rules outlined in Step 1 of the previous section, but keep in mind that you want c to be an integer. If c = an integer, then 11c must be a multiple of 11. Thus to yield an integer for c, you should choose values for a and b that will sum to a multiple of 11. Let’s choose 9 for a and 13 for b. Those values sum to 22, which is a multiple of 11.

Now use these values to solve for c:


So your values are a = 9, b = 13, c = 2.

STEP 2: Answer the question using the values you chose for the variables.

What is the average of 9, 13, and 2?


STEP 3: Plug the values into the choices and see which choice yields a value that matches your goal.

Here’s the original question again:
If a + b = 11c, what is the average of a, b, and c, in terms of a and b?
3c
3c + 1
4c
5c
5c − 1

Now substitute 2 for c in all the choices and identify which choice matches the target of 8.


The only choice that yields a value that matches the target is C.

Strategy 2: Back-Solve
Like plugging in numbers, back-solving is specific to Discrete Quantitative questions.

Back-solving is an option when the choices provide values for a variable in the question. Again, back-solving is a way to avoid algebra. Instead of working out the manipulations in the question, you can work backward to determine which choice provides a value that matches the restrictions in the question.

Let’s look at an example:

After a 20% decrease, the price of a shirt was $120. What was the original price of the shirt?
$100
$144
$150
$160
$180

Though you can certainly answer this question algebraically, let’s focus on back-solving.

STEP 1: Start with Choice B. Take the value in Choice B and determine how it relates to the information in the question. If the original price of the shirt was $144, then the new price of the shirt would be $144 − 0.2($144) = $115.20. When the original price of the shirt is $144, the reduced price of the shirt ($115.20) is less than $120. Thus $144 is too small a value for the original price of the shirt. Since the value in B is too small, the value in A must be too small as well. The correct answer is C, D, or E.
STEP 2: Back-solve with Choice D. Why Choice D? There are three possibilities: Either D is the answer, D is too small, or D is too big. If D is too small, then the answer must be choice E. If D is too big, then the answer must be choice C. No matter what happens, you won’t have to test choices C and E. Thus by testing B and then D, you ensure that you will never have to test more than two choices.
Let’s see how the value in D relates to the given information. If the original price of the shirt was $160, then the reduced price of the shirt would be $160 − 0.2(160) = $128. $128 is greater than the reduced price in the question. You can thus infer that $160 is too large of a value for the original price of the shirt. The value in Choice D is too large. If the value in Choice D is too large, then the value in Choice E must be too large as well. The correct answer must be C.