IR Explained: Q24: Earthquake Attributes

August 3, 2012

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.

The introductory paragraph provides plenty of background information on this Table Analysis question: the definition of Greenwich Mean Time, along with how longitude and latitude work. Rather than spending a minute or two with the definitions, check the questions to see what information you will need.

The three questions cover "depths," "north of the equator," and time. Depth is its own, self-explanatory column in table, and time is easy to understand, leaving only "north of the Equator." As the intro explains, latitude is zero at the equator, increasing to 90 at the north pole, so if an earthquake is north of the equator, its latitude is positive. Ignore the rest.

24A sounds very work-intensive, but as is usually the case, it doesn't have to be. If you'll need both mean and median, start with median, the easy one. Sort the table by depths, putting those numbers in order. Knowing that there are 22 earthquakes listed, find the median by averaging the 11th and 12th in the list: (25+26)/2 = 25.5.

Calculating the overall average is a daunting task, so look for a way around it. The four largest numbers in the depth column are much, much bigger than the rest. If we simply add up the three largest, we have a total of roughly 1800. Even if we ignore the other 19 depths, the average would be 1800/22, which is roughly 80--far bigger than 25.5. 24A is Yes.

It's smooth sailing from here. For 24B, sort by latitude. As we've seen, if latitude is positive, the earthquake took place north of the equator. 10 of the 22 earthqakes have positive latitude, which is less than half, so 24B is No.

For 24C, sort by time. Count the rows with a time between 10:00:00 and 20:00:00 to discover that 9 fit that description. That's less than half of 22, so 24C is No.

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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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