How to Avoid the C Trap in GMAT Data Sufficiency Questions

Last Updated on September 24, 2023

Data Sufficiency (DS) questions in the GMAT Quant section are known for their ability to twist your brain and challenge both your math and data analysis skills. One of the most challenging types of DS math questions, the so-called C trap lulls test-takers into a false sense of security because it appears to be easy and straightforward. This article provides examples of C-trap questions in DS, as well as ways to identify and work through these tricky problems.

Here are the topics we’ll cover:

• What is a Data Sufficiency Question?
• What Exactly Is a C Trap?
  • Solving for Two Variables in One Equation
• The C Trap in a DS Question
• When to Look Out for the C Trap
  • When an Equation Will Allow You to Find Unique Values for Two Variables
• Key Takeaways
• What’s Next?

Let’s start by reviewing what a Data Sufficiency question is.

What Is a Data Sufficiency Question?

A GMAT Data Sufficiency question contains a question stem and two statements (statement one and statement two). We need to use the information provided in those statements to determine whether we are able to answer the question posed in the stem.

What makes GMAT Data Sufficiency questions so different from Problem Solving questions is that, whereas in Problem Solving questions, you need to determine a numerical answer, in DS questions, you need only to determine sufficiency. In other words, you need to determine whether the information provided is sufficient to answer the question. However, you do so without having to calculate that answer.

Because you do not have to do the math “in full” for most Data Sufficiency problems, the GMAT-makers sometimes try to trick or bait you into selecting a particularly attractive answer. This is referred to as a trap. The trap we will discuss in this article is the “C trap.” 

KEY FACT:

In DS questions, you only need to determine whether you have sufficient information to answer the question; you don’t need to calculate that answer.

So, let’s talk about what exactly a C trap question is.

What Exactly Is a C Trap?

In any Data Sufficiency question, we are always provided with the same 5 answer choices:

A) Statement one alone is sufficient.

B) Statement two alone is sufficient.

C) Both statements together are sufficient, but neither statement alone is sufficient.

D) Either statement alone is sufficient.

E) Statements one and two together are insufficient.

As we see above, we select answer choice C when both statements together are sufficient to answer the question. The essence of any C trap is that C appears to OBVIOUSLY be the correct answer when, in fact, it is not. Thus, we need to be very careful not to quickly rush through the statements and hastily select C when analyzing DS questions. 

KEY FACT:

A C trap provides two statements that appear to be needed to answer the question, but in reality, either of the statements may be sufficient on their own.

Aside from taking your time and being in the moment, a good way to stop yourself from making hasty choices is to understand the mathematical reasoning behind the C trap. In one very common example of the C trap, the reasoning is that you can sometimes have two variables and only one equation, but you can still determine unique values for those variables. The fact is you likely did not learn this in a middle school or high school algebra class. However, it’s something you certainly need to know for the GMAT. So, let’s discuss this important point further.

Solving for Two Variables in One Equation

To show how to solve for two variables in one equation, let’s first practice with a Problem Solving question.

KEY FACT:

We can sometimes solve for two variables even when we are given only a single equation relating them.

We are now familiar with the fact that we can sometimes determine the values of two variables when we have just one equation. So, let’s see how this trap could play out in a Data Sufficiency question.

The C Trap in a DS Question

When we look at the following DS question, without doing any work, the seemingly obvious (but actually incorrect) answer is C.

If you smell a C trap in the air, thoroughly investigate each statement so you don’t fall for the trap.

While you should be on the lookout for the C trap, you want to avoid overthinking the C trap. Let’s discuss.

When to Look Out for the C trap

So far in this article, we’ve learned that there are specific times when one equation is sufficient to determine the value of two distinct variables. However, I don’t want you to get too carried away — not all single equations are sufficient to determine the value of two variables. For instance, if we have the equation x + y = 10, we cannot determine unique values for x and y. For example, x could be 2, y could be 8, or x could be 3, and y could be 7.

So, this raises the question: how can I tell whether a given equation will allow me to determine unique values for both variables?

When an Equation Will Allow You to Find Unique Values for Two Variables

To determine whether a given equation will allow you to find unique values for two variables, look for these two clues:

1. The variables can take on only positive integer values.
2. The integer coefficients attached to the variables are greater than 1.

Let’s review the two example equations we used in this article:

First example equation: 8w + 5d = 55, where w and d are positive integers. Recall that, in this example, variables w and d were the numbers of sandwiches and salads purchased. Thus, w and d can take on positive integer values only. Also, note that both integer coefficients in the equation are greater than 1.

Second example equation: 5x + 8y = 56, where x and y were specified as being positive integers. Also, note that both integer coefficients are greater than 1.

Equations with a “look” similar to what we see in our example equations (the variables take on positive integer values only) have the makings of C traps. So, be sure that you always investigate such equations in detail to determine whether unique values for each of the variables can be determined without needing a second equation.

TTP PRO TIP:

C-trap equations with two variables usually will have integer coefficients that are greater than 1, and the variables themselves can take on positive integer values only.

Key Takeaways

• A C trap is a DS question that lures you into prematurely selecting choice C, which states that both statements one and two are needed for answering the question posed in the question stem.
• A common C trap question provides a statement containing an equation with two variables, which appears to be insufficient for answering the question posed in the stem.
• Make sure you thoroughly investigate each statement so that you do not end up falling for the trap.
• C-trap equations with two variables usually will have integer coefficients that are greater than 1, and the variables themselves can take on positive integer values only.
• By being aware of this type of C trap DS question, you have one more tool in your arsenal for conquering the GMAT Quant section.

What’s Next?

GMAT Data Sufficiency questions are infamous for being mind-bending, difficult, and tricky. Take a look at this article for more help developing world-class skills for answering DS questions and to try your hand at some more DS practice questions just like those that the GMAT tests.

Want some more GMAT practice of key math skills and concepts? Try your hand at these 10 Quant practice questions and this free quiz on the Difference of Squares.

Good luck!

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