# Articles by Rich Zwelling

#### Is That Number Prime?

How do you figure out if a number is prime or not? We’ll discover a couple of ways to do that in today’s article, which was inspired by our students. I was recently chatting with a student about the following … Read More

#### Be Careful When Interpreting “Must Be True” Questions

If I were to ask you the general meaning of the phrase “must be true,” you probably wouldn’t hesitate. You’d simply say, “The phrase ‘must be true’ means ALWAYS true, no matter … Read More

#### The Speedy Road to Insufficiency

Here’s another quick tip for those of you still struggling with the pitfalls of Data Sufficiency. Let’s take a look at the following Official Guide DS problem: If p and q are positive integers and pq = 24, … Read More

#### Intuition and Common Sense on Geometry DS Questions

“Who cares?” That should be the operative question on your mind as you tackle Data Sufficiency problems. Here’s what I mean: Suppose I ask you the question “What is x?” I then give you … Read More

#### The AWA Isn’t That Important… So Can I Blow It Off?

I want to take a moment to address some common confusion about the Analytical Writing Assessment (AWA) portion of the GMAT exam. As many of you inveterate students know, the AWA portion involves two essays, and you get … Read More

#### Tips for Comparing Fractions on the GMAT

When discussing fractions, I often find a very interesting area of confusion among students. I’ll ask them to compare, for example, 1/3 and 1/4. Almost everyone correctly answers that 1/4 is smaller. “How do … Read More

#### Filling in Your Chart: A Data Sufficiency Primer

In last week’s post, I gave the following Data Sufficiency problem to be solved using a chart: A certain zoo has 288 mammals, 25 percent of which are female. What percent of the mammals in the zoo were born at the … Read More

#### To Venn or Not to Venn? Depends on the Question

Many students like to ask me the following question when dealing with overlapping sets problems: “Which are better? Venn diagrams or charts?” I’ve found that students like to ask because every student … Read More

#### Two Shortcut Formulas for Set Problems

If you’re one of those people who goes crazy over overlapping sets problems, this post is for you. Oftentimes, Venn Diagrams are the way to go on these problems, and I highly recommend that you master them in our … Read More

#### More Fun with Weighted Averages

In last week’s post, I discussed how weighted averages are often tested conceptually and thus show up on Data Sufficiency problems. However, even when weighted average problems appear on the more results-oriented … Read More

#### What the Heck is a Weighted Average?

If you’ve done some GMAT preparation already, you’ve likely come across the concept of “weighted averages.” But what does that term really mean? In short, the term “weighted” is simply … Read More

#### Quick Mental Math: Averages

I run the risk of sounding like a hypocrite here. I’m often fond of telling my students that the GMAT is a reasoning test, not a speed calculation test. (I say this to assuage their fears after I break the bad … Read More

#### How to Approach Rate Problems on GMAT Quant, Part II

Last week, I focused on rate problems involving speed (if you’re looking to the answer to last week’s challenge question, scroll down to the end of this post!). This week, I’m going to shift to … Read More