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GMAT Tips: How Do You Know That? Or: The Importance of “Why”

Today’s GMAT tip comes from test prep firm ManhattanGMAT. In this article, they explain the importance of understanding how or why you know an answer when studying for the GMAT.  Read on to see what they have to say!

Today’s article is going to be a little bit different from our usual. Rather than picking apart one question or talking about how to review tests or fix weaknesses – that is, pretty specific things – we’re going to address a “meta” topic. When you’re doing problems, when you’re studying, when you’re discussing something with friends or your teacher, what’s that one crucial thing that you should be asking yourself?

The answer is in our title: How do you know that? Or, more simply: why?

We spend most of our time and energy figuring out what to do. What calculations are necessary to solve this problem? What grammar rules do I need to apply? What kind of logical thinking do I need to do on this CR problem?

After pretty much every single “What” question, your next question should be “Why?” or “How do I know that?”

Let’s say I decide (or, possibly, my book or teacher tells me) that the best way to answer a certain probability question is to use the “1 – x” shortcut. (This shortcut allows me to calculate the probability for the outcome that I don’t want, and then subtract that from 1 to get the probability for the outcome that I do want. We’ll talk more about this below.)

How do I know that? Let’s see – oh, it’s because the question asks for the number of outcomes that will include “at least” one head. The “at least” language is a trigger, a clue, that the 1 – x shortcut will likely work on that problem.

Why? Why does that work the way it does? Let’s say that the problem asked me to flip a fair coin (50% chance of landing on heads, 50% chance of landing on tails) four times and calculate the probability that the outcome will include at least one head.

What are the possible outcomes in general (ignoring probabilities of each)? I could have zero heads (that is, all tails). I could have 1 head. I could have 2 heads, or three heads, or four heads. If I calculated the probabilities of each of those, what would the sum of those probabilities be?

# of headsprobability
0 heads?
1 head?
2 heads?
3 heads?
4 heads?
Sum of all outcomes1


The sum of the probabilities of all of the possible outcomes equals 1. Now, which probabilities do we want and which don’t we want?

The question asked us for the outcomes that include at least one head. So we need to include the probabilities for: 1 head, 2 heads, 3 heads, and 4 heads. In fact, there’s only one probability category that we don’t want: 0 heads. That’s interesting. Call that probability x. What does the 1 – x shortcut really mean?

If all of the probabilities add up to one, and if they ask for all of the probabilities except for the “all heads” outcome, then I can subtract the “odd one out” from 1 and have my answer! That’s much easier than having to calculate the individual probabilities of each of the outcomes that I do want.

So, when I have a probability question, my trigger that I should think about using the 1 – x shortcut is some language about “at least” or “at most” – that is, something that might be setting me up to have an “odd one out” situation. Then I just have to check to see whether I really do have an “odd one out” setup on the problem (and, this being the GMAT, that will be true the vast majority of the time that I see “at least” or “at most” language on a probability question).

None of that is about how to do the math – it’s all about WHY this approach works in the first place and HOW I KNOW to use the technique on any particular problem. This is crucially important when studying for standardized tests. If you only study what to do, how will you know when to use all of these great things that you’ve learned?

What does all of this mean? It means that you need to add two standard questions to your study repertoire: Why? How do I know that? When you are studying with someone else (whether a teacher or a fellow student), ask these questions constantly. Whenever someone’s explaining something to you, don’t settle for understanding what the person is telling you (though obviously that is important, too!). Also understand why this problem works the way it does, how you (or your teacher or your fellow student) knew that in the first place, and how you could recognize something similar on another, future problem.

Here are some additional resources that can help you with this process:

Disguising and Decoding Quant Problems – an article that talks about how the GMAT disguises things and how to go about figuring out what they’re really trying to tell you.

How to Analyze a Practice Problem – a series of questions to ask when reviewing practice problems after you’ve already tried them

How to Learn From Your Errors – first question: why, specifically, did I get this wrong?

Finally, consider taking an index card or piece of paper, writing “WHY?” on it in big letters, and taping it up in your study area (or just laying it out when you start studying). It seems simple – and it is, in one sense – but it’s also very hard to push yourself consistently on this question. If you can train yourself to ask “Why?” and “How do I know that?” every step of the way, you’ll get a lot more out of your GMAT study!

For more information on ManhattanGMAT, download Clear Admit’s independent guide to the leading test preparation companies here. This FREE guide includes coupons for discounts on test prep services at ten different firms!

Posted in: GMAT Tips

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