Hi All,

Regardless of how you might feel about the Quant section of the GMAT, it's worth noting that the GMAT is NOT a ‘math test’ – it’s a ‘critical thinking test’ that uses math as the subject through which you can prove your critical thinking skills. While the Quant section does require that you complete lots of basic calculations as you work through it, the GMAT will NEVER require that you complete complex calculations to get to the correct answer… so if you CHOOSE to approach questions in that way, then you will likely limit how high you can score. If one of your goals is a Q51, then you would find it really helpful to build up a multitude of Quant skills, instead of focusing on complex, long-winded ‘math approaches’ that often take longer to implement than other more-strategic options (and inherently increase the chances of you making little mistakes along the way).

The GMAC Official Guide is a fantastic book of practice questions – and if you don’t have a copy, then you should absolutely purchase one for your studies. Consider the following question that appears in the Diagnostic Test of the GMAC Official Guide. It’s question #8 in the Problem Solving section of the Diagnostic – and has appeared in the last several versions of that book:

If a certain toy store’s revenue in November was 2/5 of its revenue in December and its revenue in January was ¼ of it’s revenue in November, then the store’s revenue in December was how many times the average (arithmetic mean) of its revenues in November and January?

1/4

1/2

2/3

2

4

Final Answer:

This is a fairly mid-level prompt; it’s a little wordy but the ‘math’ behind it isn’t too difficult. How would you approach it? Would you use algebra? Do you recognize that there are at least two OTHER ways to get to the correct answer (and one of those approaches requires almost no math…)?

Take a moment to answer this question in whatever way you choose. Write everything down and then compare your approach to the options listed below. Would one of the other options potentially have been faster or easier for you....?

.

.

.

1st Approach: Algebra

The explanations provided in

the Official Guide often focus on the ‘math approach’ as that is that standard approach taken in most math books. However, that type of approach is often step-heavy and arguably takes the longest to complete.

For this prompt, we can create 3 variables:

N = revenue in November

D = revenue in December

J = revenue in January

With the information in the first half of the prompt, we can create two equations:

N = (2/5)(D)

J = (1/4)(N)

We’re then asked to calculate (D) / [(N+J)/2]

With the given equations, we can translate each variable ‘in terms of’ D and plug in… Again, this is tedious, step-heavy work, but here goes...

J = (1/4)(2/5)(D) = (2/20)(D) = (1/10)(D)

With the above values for N and J, we have the following fraction to simplify:

D / [(2D/5 + D/10)/2]

D / [(4D/10 + D/10)/2]

D / [(5D/10)/2]

D / (5D/20)

D / (D/4)

4D / D

4

2nd Approach: TESTing VALUES

Many questions in the Quant section can be solved with a Tactical approach (and part of your training should focus on learning those approaches and when certain Tactics are applicable). Here, we are given no actual values to work with, so we can choose our own.

Based on the fractions involved, the common denominator would be 20, so let’s start with D = 20…

IF….

D = 20 then

N = (2/5)(D) = 8

J = (1/4)(N) = 2

Now, we just have to place D = 20, N = 8 and J = 2 into the question….

(20) / [(8 + 2)/2] = (20) / (5) = 4

This IS the answer to the question - and you’ll get that same result regardless of the values that you choose to TEST (as long as your numbers ‘fit’ the given equations). This approach has the benefit of being fast and easy (consider the work involved - we’re really just adding and multiplying small numbers together). With fewer steps, you’re also less likely to make a little mistake along the way.

3rd Approach: Logic (and using the ‘spread’ of the answers to your advantage)

Certain questions in the Quant section are designed with really fast ‘concept shortcuts’ in mind. This is done on purpose to reward strong critical thinkers. Business Schools are looking for EXACTLY that type of critical-thinking applicant, but the GMAT has no way to give you extra points for ‘being clever’ – it can only provide you with potential shortcuts that will save you time and decrease whatever pacing-related anxiety that you might have. As a result of finding these shortcuts, your chances of scoring higher should increase, since you’ll have more time to answer the remaining questions than someone who is using lengthy math approaches and losing time as a result.

Here, consider how the given monthly revenues relate to one another….

1 - November’s revenue is 2/5 of December’s revenue. This means that the December revenue is MORE THAN DOUBLE November’s revenue.

2 – January’s revenue is ¼ of November’s revenue. Since January’s revenue is so much smaller than November’s revenue – and we already know that December’s revenue is MORE THAN DOUBLE November’s revenue, then this means that December’s revenue is a LOT greater (FAR MORE than double) than January’s revenue.

We’re asked to compare December’s revenue to the AVERAGE of November’s and January’s…. Averaging those two smaller numbers will lead to a result that is SMALLER than November's. By extension December’s revenue will be far GREATER than double than average. Looking at the answer choices, there’s only one answer that fits that description…. 4.

Now, consider how much actual work went into the 2nd and 3rd approaches to this prompt (especially relative to the work that went into the 1st approach). Assuming that you had an equal ability to tackle this question using all 3 approaches, which one would be fastest and easiest to implement? If you’re training to be a 760+ Assassin, then you know that it’s NOT the 1st approach.

GMAT Assassins aren’t born, they’re made,

Rich

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# Rich Cohen

Co-Founder & GMAT Assassin

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