Electric Lemonade: Using Personal Knowledge on the GMAT

March 7, 2012
Eli Meyer

GMAT BlogThe old saying goes, “when life gives you lemons, make lemonade.” I prefer: “when life gives you lemons, make batteries.”

About two-thirds of people who hear that joke stare at me blankly, so don’t feel bad if you have no idea what I’m talking about. Many of my friends, however, are nerdy enough to recognize this reference to the lemon battery, a classic grade school science experiment.

Little bits of trivia like this are one of my favorite things in the world, and I’ve read books and memorized minutiae about topics from Archaeology and Zoology. This is very helpful when it comes to watching Jeopardy and playing pub trivia (my wife and I won just last week!), but it isn’t as useful as you think when it comes to the GMAT.

The GMAT is designed to be taken without any specific knowledge requirements. Of course, a neuroscientist may have an easier time with a passage on neurons, and an anthropology major will have a moderate advantage reading about the living habits of the Cherokee. But although background knowledge can provide a framework for understanding a complex text, it can also lead you astray.

Here’s how: the GMAT expects all of your answers to be based on the passage. It doesn’t matter if an answer choice is true or logical. The testmaker is testing your ability to read and comprehend new material, not your ability to retain trivia from college. So watch out: some trap answers deal with common assumptions or misconceptions that the passage, in fact, dismisses. Others are “true” in the sense that they are based on modern scholarly opinion, but are out of the scope of what the passage actually discusses. The tricky answer choices prey on ‘experts’ who get overconfident about the subject of their expertise.

For instance, let’s pretend that Wikipedia’s article on lemon batteries is a RC passage. Without reading, I know that the common lemon battery uses copper and zinc. But if I had picked an answer choice that said, “lemon batteries require copper and zinc,” I would be wrong; Wikipedia really tells me that magnesium is a more effective (if more toxic) substitute. I also know with certainty that the lemon battery is very common in elementary schools—but I would be wrong if I chose any answer that mentioned primary schooling, because even though that answer might be factually correct, the article itself mentions only “science textbooks,” without specifying a grade level.

Fortunately, this trap is easily avoided with one simple strategy: read the text. Make sure that your answers are consistent with the information provided to you, and be careful not to fill in missing information until and unless a question asks you to do so. In today’s question of the week, one answer choice traps a lot of students because, based on their knowledge of the world and of the security industry, it seems obviously true. But it fact, it’s a trap—it isn’t mentioned in the passage, so it can’t be the correct answer. As you solve this question, see if you can spot the misleading trap answer as well.  Good luck!


A new electronic security system will only allow a single person at a time to pass through a secure door. A computer decides whether or not to unlock a secure door on the basis of visual clues, which it uses to identify people with proper clearance. The shape of the head, the shape and color of the eyes, the shape and color of the lips, and other characteristics of a person’s head and face are analyzed to determine his or her identity. Only if the person trying to open a secure door has the required clearance will the door unlock. Because this new system never fails, an unauthorized person can never enter a secure door equipped with the system.  If the statements above are true, which of the following conclusions can be most properly drawn?

(A) The new system is sure to be enormously successful and revolutionize the entire security industry.

(B) The new system can differentiate between people who are seeking to open a secure door and people passing by a secure door.

(C) No two people have any facial features that are identical, for example, identical lips.

(D) High costs will not make the new security system economically unviable.

(E) The new computer system is able to identify some slight facial differences between people who look very similar, such as identical twins.



Step 1: Identify the Question Type

Since the stem asks us to accept the statements as true and draw a conclusion on the basis of them, this is an inference question.

Step 2: Untangle the Stimulus

The stimulus tells us that a new electronic security system is completely failsafe and will never allow an unauthorized person through a door equipped with the system. And the system allows an authorized person to enter solely on the basis of the person’s appearance and facial features.

Step 3: Predict the Answer

Attempting to predict the correct inference could waste time, but on the GMAT, to make an inference means to determine what must be true, not just what could or might be true. It’s crucial to approach the answer choices with this in mind.

Step 4: Evaluate the Choices

(A) is out of scope. We have no evidence of how the security industry is going to respond to the new system.

(B) doesn’t need to be true. The new system doesn’t need to differentiate between people passing by the door and people trying to enter, as long as it lets authorized people in and keeps unauthorized people out. (C) is too extreme.  We don’t know that any one feature cannot be the same.  All we know is that all of the features can’t be the same.  According to the stimulus, the security system examines multiple facial features to determine identity.

As for (D), costs are outside the scope of this stimulus, since the stimulus only discusses the likelihood that unauthorized people will be able to get past the security system and through a secure door. That means the correct inference must be (E): if one twin is authorized and the other isn’t, we know the door must be able to tell them apart, because the stimulus tells us that the security system never fails. Thus, (E) must be true.

Eli Meyer Eli Meyer has been a Kaplan teacher since 2003. He has spent the past four years focused almost exclusively on the GMAT, and also has prior experience helping students ranging from middle-schoolers taking the ISEE to professors retaking the GRE for their second PhD. During his Kaplan career, Elis has also written and revised Kaplan course materials and acted as a community liaison on several popular GMAT message boards, all the while helping his students succeed both in and out of the classroom.

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