How to Spot Traps on GRE Quantitative Comparison Questions

By Ron Woldoff, Joseph Kraynak

When you take the GRE, you’ll need to watch the Quantitative Comparison (QC) questions because they have many tricks and traps. The following tips will help you in many instances.

Whatever the information, the answer choices for QCs are always the same:

(A) Quantity A is greater.

(B) Quantity B is greater.

(C) The two quantities are equal.

(D) The relationship cannot be determined from the information given.

Equal appearances can be deceiving

If Quantity A and Quantity B appear to be equal at first glance, don’t fall for it — a trap is almost always involved. Check out the following example:

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Your gut reaction may be to choose Choice (C) because both quantities are equal. After all, wasn’t it drilled into your head in school that pi equals 3.14? You may be very pleased that you remembered the numerical value of pi.

But hold the phone: The value of pi is only approximately 3.14. The number

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is actually slightly larger than that and continues as a nonrepeating, nonterminating decimal: 3.141592…, making it greater than 3.14 and Choice (A) the right choice. Correct answer: Choice (A).

Figures may not be drawn to scale

If a figure in a QC problem isn’t drawn to scale, use other information to try to solve it. Even if the drawing contains the caveat, “Note: Figure not drawn to scale,” telling you that it’s way off, you can still gather information, such as line lengths or angle measurements, that enable you to answer the question. Avoid the temptation to eyeball figures to arrive at your answer.

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Sure, x and y each appear to be 45 degrees, or roughly 45 degrees, but because you have no other information, you can’t make an estimate. You can’t deduce that x and y are equal. You do know that x and y add up to 90 degrees, because angles along a straight line add up to 180 degrees, and you already have a right angle (signified by the tiny box):

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But you don’t know how much of the 90 is x and how much is y, so go with Choice (D). Correct answer: Choice (D).

Figures may be deceptive

The GRE may provide pictures (figures) to assist you in visualizing what the question is asking, but it may also use these pictures as traps. These drawings are almost never to scale. The GRE is challenging your grasp of the concept, not your ability to eyeball the lines and angles.

Two types of drawings are always to scale: the data graph and the coordinate grid (with x and y axes). Other than that, if the GRE doesn’t tell you the shape is a square, you can’t assume the angles are 90 degrees or the sides are equal.

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Don’t choose Choice (D) just because you don’t have measurements of the triangle. Though the image isn’t drawn to scale, you can still estimate what you need in order to answer the question and compare the quantities.

The formula for the area of a triangle is

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What’s the base? You don’t know, but you do know that it’s less than the base of the rectangle because the sides of the triangle don’t extend all the way to the sides of the rectangle. Whether the drawing is to scale or not, you know that the base is less than 20. What’s the height? You don’t know that either, but you do know that it’s less than 10. Multiply less-than-10 by less-than-20 to get less-than-200. Half of less-than-200 is less-than-100. Because less-than-100 is smaller than 100, choose Choice (B). Correct answer: Choice (B).