GMAT Tip of the Week: Beware of (Richie) Incognito Information

If you’ve been following the strangest story to hit the NFL since Manti Te’o did, you’ve probably noticed that Richie Incognito is nowhere near incognito. There’s nothing subtle or understated about the guy. He’s Rob Ford in a different jersey. But there’s something about that name…

While you don’t have to fear Richie Incognito on the GMAT, there is a little bit to fear about the bullying you could receive from a different kind of “Incognito”. The GMAT – and in particular Data Sufficiency – loves to bully you with incognito information. Consider these two questions:

The swimming pool at Jonathan’s house can contain up to x gallons of water. How many gallons does the pool hold when completely full of water?

(1) x^2 = 160000

(2) 399 < x < 401

and

The aquarium at Stephen Ross’s house can contain up to y dolphins. How many dolphins does the aquarium hold when completely filled with dolphins?

(1) y^2 = 160000

(2) 399 < y < 401

Those questions look the same, right? It’s just that the second has a slightly goofier backstory, but other than that what’s the difference?

Much to Jonathan’s and Mr. Ross’s fears, Incognito appears in both of them, twice in the second one.

For statement 1 in each case, taking the square root of both sides gets you to either 400 or -400, which even to a rookie GMAT student being hazed by tough practice questions screams “beware the negative! Insufficient!” But wait – where’s Incognito? Neither question tells you specifically that the variable has to be positive, but incognito information guarantees it. You can’t have a negative amount of water in a pool, and you can’t have a negative number of dolphins in a pool (although as Mr. Ross knows, you can have negative Dolphins on your team so you need to police that locker room). So in each statement 1 the information is sufficient. Only 400 is a plausible answer.

In Question 2, Statement 2, Incognito strikes again. In the first question, it’s certainly possible to have 399.5 gallons of water. But you can’t have 399.5 dolphins. In subtle, incognito fashion the backstory in the second questions guarantees both “positive” and “integer”, making the answer to the second question D while the first is A. And in either case, what looked like just a plain backstory behind the algebra was actually quite important to the answer – it was important definitions (positive, integer) masquerading as window dressing. It was the GMAT gone Incognito.

What can you learn from this? Make sure to be on the lookout for incognito information, which can include:

-In Geometry questions where exponents are present, you can’t have negative lengths or volumes. Geometry in its incognito way rules out negative.
-When the units they choose can’t be divided smaller than integers, you have incognito positive and integer information.
-Ratio problems are famous for incognito information. If the ratio is 2:3, the total number will be a multiple of 5; you can derive an extra multiple just from the individual components. (but wait – if there’s possibly a third component to the ratio, then that hidden possibility ruins the total/multiple trick)
-In Venn Diagram problems, the “Neither” component also often travels incognito. If you get the information “10 people are in group A, 12 are in group B, and 20 are out there total”, you’re tempted to say that 2 people are in both…but “Neither” is lurking there all unsuspecting and incognito on you.

There are other examples, but the main lesson is this – the GMAT thrives on Incognito bullying. It will punish you by hiding important information in disguise, so be on the lookout for Incognito.

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By Brian Galvin