IR Explained: Q27: Building Floors and Heights

August 10, 2012

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This post is part of a series--IR Explained--that walks through the sample Integrated Reasoning questions provided in the latest edition of the GMAT Official Guide.

On some Graphics Interpretation questions, the graph will not be intuitive. In those cases, spend a little more time reading the labels on the graph and the introductory paragraph so that you can gain a thorough understanding of the data.

In this case, the graph includes two dots--one red, one black--for each of several buildings. Each building has a height between 350 and 510 meters. That measurement is the x-axis.

There are two y-axes. On the left, we have "number of floors," and each building has a red dot that reflects the number of floors given the building's height. For instance, the red dot is the far lower left tells us that a building roughly 355 meters high has approximately 53 floors.

The black dots reflect the "mean height per floor," which is the y-axis on the right. The black dot that refers to that same ~355-meter-high building is near the top of the graph, indicating that that building's mean height per floor is just above 6.5 meters.

The intro says there are 22 buildings. Thus, the graph contains 22 black dots and 22 red dots--one of each for each building.

27A asks you identify the building with the greatest mean height per floor. Mean height per floor is the black dot, and the highest black dot is near the upper left corner of the graph, corresponding with a roof height of about 365 meters. Thus, the height of this building is between 350 and 370 meters.

27B refers to a concept that you may not be familiar with: correlation. There are three choices: strong negative, negligible, and strong positive. If a correlation is positive, it means that when one attribute is high, the other is generally high, and vice versa. When a correlation is negative, if one attribute is high, the other is low, and vice versa. A negligible correlation refers to situations where there is no consistent relationship between the two attributes.

In this example, the correlation is strong negative. For most of the buildings with roof heights of 410 and above, the red dots and high and the black dots are low. For two of the shortest buildings, the black dots and high and the red dots are low. There are a few instances where both are in the middle, but in general, one attribute is high and the other is low.

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About the author: Jeff Sackmann has written many GMAT preparation books, including the popular Total GMAT Math, Total GMAT Verbal, and GMAT 111. He has also created explanations for problems in The Official Guide, as well as 1,800 practice GMAT math questions.

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