Estimation Assumption on the GMAT test is a critical skill. The video below will teach how to estimate well and how to ace the gmat math section.
Estimation Assumption on GMAT Test [Video Transcript]
In this lesson, we will examine some assumptions you can make with geometry questions, and some that you cannot make. In an earlier lesson, we examined questions similar to this one. As you may already know, we can deduce that the other angle here is 60 degrees. Please note that this calculation is based on the assumption that this angle here is 180 degrees. But is this a reasonable assumption? Perhaps there is a very, very slight bend at the point and this angle is actually 179.99 degrees. If that is the case, then the conclusion that the missing angle here is 60 degrees, is incorrect. Now fortunately on the GMAT, all lines that appear to be straight can be assumed to be straight. So we can correctly deduce here that the missing angle is 60 degrees.
Okay. Now, let’s talk about specific angles. If we are given an angle such as the one shown here, what assumptions can we make? Well, this angle certainly looks like a 90-degree angle. But can we assume that it is 90 degrees? The answer is no. When it comes to angle measurements, we cannot make any assumptions. Angle X could be 90 degrees, or it could be 89.999 degrees. Or, it could be 10 degrees. There is no way to tell here. Okay. What about this example? What assumptions can we make about angles X and Y? Can we assume, for example, that the two angles add to 180 degrees? Well, yes, we can conclude this since the line does appear to be straight, in which case the two angles are on a line which means they add to 180 degrees.
What about this? Can we assume that both angles here are greater than zero degrees? The answer is also yes. If an angle is shown in a diagram, then we can assume that that angle is greater than zero degrees. Okay. Now, let’s talk about parallel lines. We cannot assume any lines are parallel unless we are specifically told so or we have been given information that proves the lines are parallel. So we must not assume lines are parallel merely because they appear to be parallel.
At this point, let’s talk a little bit more about the figures that sometimes accompany geometry questions. For problem solving questions, please note that all figures are drawn proportionately unless the question explicitly states that the figure is not drawn to scale. If a figure is drawn to scale, you can estimate angles, lengths, and areas to confirm your calculations or to help guide your guesses if you are unsure how to solve the question.
For example, let’s say you were tackling this question which asks us to find the value of X. If you do not know how to find this angle, and you are forced to guess, you can probably eliminate some answer choices by estimating the angle. Conversely, if you were able to calculate the angle, your visual estimation of the angle may help confirm your calculations.
Now later in this module, we will be tackling a very similar question. So I won’t tell you the answer at this time. Now, please keep in mind that while visual estimation can sometimes help you with a geometry question, the test makers are very careful to ensure that very few problems can be solved merely by visual measurement or estimation. It is always much wiser to use your knowledge of geometric properties to solve problems. In fact, to encourage mathematical deduction over estimation, most of the practice questions in this module are intentionally not drawn to scale.
Now, when it comes to data sufficiently questions, the accompanying figure will always conform to the information in question. However, the figure does not necessarily conform to information in statements. Now, since that is sufficiency questions, require you to determine whether the statements provide sufficient information, you might have to rely solely on the information in the statements.
Visual estimation can lead you to make conclusions that are not supported by the statements, so avoid using any visual estimation for data sufficiently questions. Okay. To summarize, in this lesson we learned that lines that appear straight can be assumed to be straight, all angles are greater than zero degrees, never make assumptions about angle measurements, never make assumptions about parallelism, and be sure to use visual estimation very sparingly.