In the figure above, if MNOP is a trapezoid and NOPR is a parallelogram, what is the area of triangular region MNR? GMAT Explanation

In the figure above, if MNOP is a trapezoid and NOPR is a parallelogram, what is the area of triangular region MNR?

(1) The area of NOPR is 30
(2) The area of the shaded region is 5

When I first saw this question on Exam Pack 1 I didn't think much of it. Strangely though it comes up a ton in GMAT tutoring sessions even with students who are solid on the quant. The name of the game: stay organized, make the easy inferences, and avoid any assumptions/fake rules.

What are we looking for? The area of triangular region MNR

OK - easy. You know the formula for the area of a triangle right? So start there and think about what blanks you are going need to fill in.

Anything we can add to the figure?

Yes! Both a parallelogram and a trapezoid have symmetry that we can use to our advantage. Also, MNR has two angles the same so the sides opposite those angles must also be the same.

If you drop an altitude at N you'll split MNR in half (because of the two equal angles). Each of those halves is equal to the triangles QOP. You can go further and add a line at O dropping down to line MP and you'll have two equal triangles on either side of the figure. I added all of this to the diagram below.

In the figure above, if MNOP is a trapezoid and NOPR is a parallelogram, what is the area of triangular region MNR? Diagram

(1) The area of NOPR is 30

Knowing the area of parallelogram doesn't tell us the dimensions of the parallelogram or more importantly the Base/Height of the triangle. We can make an infinite number of different parallelograms with area 30. Those variations will create different areas for the triangular regions.

You can think of this at the extremes. Let's say parallelogram NOPR had a base of 1000 and a height of .03 (area of 30). And the parallelogram only has a very slight angle to it. The triangle will have a tiny area. You can push things in the other direction as well. Give NOPS a height of 1000 and a big angle. The triangle will have a much greater area. You can see in the diagram below that depending on the dimensions and angle of the parallelogram you can make a bigger triangle with a smaller parallelogram.

In the figure above, if MNOP is a trapezoid and NOPR is a parallelogram, what is the area of triangular region MNR? Diagram

So to sum it up: just because you know the are of a parallelogram doesn't mean you know the area of the triangles in a parallelogram.

INSUFFICIENT.

(2) The area of the shaded region is 5

If we know the area of the shaded region then we can just double that to get the area of triangular region MNR. SUFFICIENT.

Correct Answer: B

More GMAT Data Sufficiency Geometry Practice Questions!

This is from the GMAT Official Guide. It's a tough one that has both an algebraic component and a "distortion"/testing extremes component which is similar to what we did for statement 1 above (to prove insufficiency). In the figure above, is the area of triangular region ABC equal to the area of triangular region DBA?

Here are two similar-ish DS Geometry questions from GMAT Question of the day:

GMAT Question of the Day Data Sufficiency Geometry 1 

GMAT Question of the Day Data Sufficiency Geometry 2 

 

In the xy-plane, at what two points does the graph of y = (x + a)(x + b) intersect the x-axis? GMAT Explanation

In the xy-plane, at what two points does the graph of y = (x + a)(x + b) intersect the x-axis?

(1) a + b = -1

(2) The graph intersects the y-axis at (0, -6)

 

Correct Answer: C

In the figure above, is the area of triangular region ABC equal to the area of triangular region DBA? GMAT Explanation

In the figure above, is the area of triangular region ABC equal to the area of triangular region DBA? diagram

In the figure above, is the area of triangular region ABC equal to the area of triangular region DBA?

(1) (AC)^2 = 2(AD)^2

(2) ∆ABC is isosceles.

This is a tough GMAT Data Sufficiency question from the Official Guide. We'll be attacking it with Yes/No and distortion. But first let's make sure to read carefully and define the question. Get a sense for the diagram and make some inferences. We want to know whether the area of triangular region ABC is equal to the area of triangular region DBA. We have two right triangles so let's keep in mind that right triangles have certain properties and could be special right triangles, 45-45-90 (right isosceles) or 30-60-90 or some special ratio 3-4-5, 5-12-13.

Not much to do with the diagram so let's jump into statement (1) (AC)^2 = 2(AD)^2

We can pick some values that follow that ratio. Let's make AD = 1 so AC must be √2. Now let's go ahead and do a Yes/No. You might think, hmmm, there are a million value I could pick. This is getting confusing... That's true. However, let's keep it easy. Especially because you know that you have two right triangles. Use special triangles as the side ratios will be defined for you. Also, GMAT loves special triangles so they are often a great bet for scenarios on GMAT DS. So let's try a 45-45-90 configuration and see where that takes us and then a 30-60-90 and see if that agrees or not.

If AC = √2 then CB also equals √2 leaving the area for triangular region ABC at 1. Because of 45-45-90 (or Pythagorean theorem) AB equals 2. So, with AD = 1 and AB = 2 the area of triangular region DBA is also 1. So in this case the area of triangular region ABC  is equal to the area of triangular region DBA. That gives us a clear "Yes". But that's only one scenario. For GMAT DS we have to try to prove insufficiency. Now let's see what happens with a 30-60-90.

AC = √2 CB = √6 so the area is √12/2

AD = 1 AB = 2√2 (because of 30-60-90 rules) so the area is 2.

With a 30-60-90 example the area of triangular region ABC  is not equal to the area of triangular region DBA. Because we proved a "yes" and a "no" statement 1 is insufficient.

Let's take a look at statement (2)∆ABC is isosceles.

For this one we can use "distortion". Keep triangular region ABC the same but see if you can change the size of triangular region DBA. You'll notice that you can grossly distort DBA clearly changing the area. Because you can change the area of DBA while leaving ABC unchanged statement (2) is insufficient.

Now let's put the statements together. Statement (2) constrains us to 45-45-90 and statement (1) tells us that if the triangular region ABC is a 45-45-90 then the area of triangular region ABC is equal to the area of triangular region DBA. We have a clear "yes" and there are no other possibilities so together the statements are sufficient. The answer is C.

In the figure above, is the area of triangular region ABC equal to the area of triangular region DBA? full solution diagram

There are two GMAT Data Sufficiency strategies we worked on here:

  1. Yes/No. With a question that asks for a yes or no answer organize your solution into Yes/No. Write Yes and No on your paper to remind yourself that you need to try create scenarios that prove a yes and a no.
  2. Distortion. This can be very helpful for getting through tough geometry questions very quickly. Try to manipulate the shape while respecting whatever constraints there are in place. If you can shift the size of one shape vs another or one angle vs another you can easily prove insufficiency without doing any calculations.

Additional GMAT Data Sufficiency Yes/No, Distortion, Geometry Practice Questions

Here are a bunch of DS Yes/No examples. A couple are very similar a couple are somewhat different but for all of them you can used either Yes/No or Distortion. I'd work through all of these to master those concepts.

Here is another challenging geometry question with right isosceles triangles: The perimeter of a certain isosceles right triangle is 16 + 16√2. What is the length of the hypotenuse of the triangle?

Here's a very similar Geometry Yes/No Distortion question from GMAT Question fo the Day

Here's another GMAT Question of the Day Geometry Distortion example.

And one more GMAT Question of the Day Yes/No example.

Here's an Official GMAT question with Yes/No strategy: In the xy-plane, region R consists of all the points 

This GMAT Question of the Day Geometry example isn't really a Yes/No question because you are looking for a specific value but you can still use Yes/No and Distortion to prove insufficiency

Also, Yes/No isn't just for Geometry. Here's a Yes/No divisibility example from the GMAT Official Guide

In the xy-plane, region R consists of all the points (x,y) such that 2x + 3y ≤ 6. Is the point (r,s) in region R? GMAT Explanation

In the xy-plane, region R consists of all the points (x,y) such that 2x + 3y ≤ 6. Is the point (r,s) in region R?

(1) 3r + 2s = 6

(2) r ≤ 3 and s ≤ 2

This is a tough one and it's rare that one our GMAT tutoring students, even the quant inclined, get this right on the first shot. First, read carefully and figure out the question. You might ask yourself: I've got x,y in the inequalities but r,s in the question. What's that about? Well, r,s correspond to x,y. r = x, s = y. Easy. Basic idea, we need to see if r, s is within the graph's boundaries. In the statements we're given constraints that we need to account for.

Let's go ahead and graph the given inequality. Use the x and y intercepts (make x = 0 then make y = 0). If you need a refresh here's a Khan video on graphing inequalities.

2x + 3y ≤ 6

In the xy-plane, region R consists of all the points (x,y) such that 2x + 3y ≤ 6 GMAT Explanation

That's really half the battle right there as, again, you'll just be plotting points to see if you're in or out of the graph's boundaries.

So let's start with statement (1) 3r + 2s = 6.

Use easy numbers. Make r = 0 then s = 3. 0,3 is outside of the graph (region R) so that gives us a "no". Now let's try for a "yes". Let's try just flipping things around making s = 0 so r = 2. That's on the graph so we also have a "yes". That makes statement (1) insufficient.

In the xy-plane, region R consists of all the points (x,y) such that 2x + 3y ≤ 6 GMAT Explanation 2

We can follow the same procedure for statement (2), trying to prove a "yes" and a "no".

(2) r ≤ 3 and s ≤ 2

Try to re-use examples from the other statement as that tends to save time. We can still use our "yes" with s = 0, r = 2. Our "no" won't work because s ≤ 2. So we need to consider whether we can create a know given this constraint.

Why not just try the upper limits to try to push out of the graph? r = 3 s = 2. Clearly outside of the graph so we have a "no". That makes statement (2) insufficient.

In the xy-plane, region R consists of all the points (x,y) such that 2x + 3y ≤ 6 GMAT Explanation 3

Let's put the statements together and, if possible, reuse your examples that satisfy both statements.

(1) 3r + 2s = 6

(2) r ≤ 3 and s ≤ 2

We can still use our yes example because it fit both constraints, s = 0, r = 2. Now we need to think about a "no". Think easy. Again, I would aim for the extremes and see where that takes us. Let's make s = 2, the upper limit for s, and see what happens. r = 2/3. Looking at the graph it's clear that with r = 2/3 that s = 2 is out of bounds giving us a "no". Even with both statements we're insufficient.

In the xy-plane, region R consists of all the points (x,y) such that 2x + 3y ≤ 6 GMAT Explanation 4

A few big picture GMAT strategies to consider:

  1. Yes/No for Data Sufficiency. This is a biggie. Yes/No is a critical GMAT Data Sufficiency strategy. It helps you organize the question and focuses your number picking. Your goal is try to prove insufficiency!
  2. Sometimes GMAT tutoring clients ask: how do I know what values to pick? I get lost, end up trying out a million different options, waste a ton of time, and then just guess anyways. Use the question to help define the values. Look for endpoints or extremes. Try to consider values that will clearly fall on the "yes" or "no" side. If lost try something and consider the result.

Here's a video solution:

https://youtu.be/n1j-DFRqsmI

Additional GMAT Data Sufficiency Yes/No Practice Questions

Here's another Yes/No DS question. It's not geometry but, again, the strategy is Yes/No so the underlying content can change but the strategy remains the same: A school administrator will assign each student in a group

Here's a Geometry Yes/No question from the GMAT Question of the Day.

And another challenging Geometry GMAT Question of the Day

Here's an Official GMAT Question that has very different Geometry content but uses the same Yes/No strategy: Is the area of triangular region ABC equal to the area of triangular region DBA

 

 

 

 

 

In the figure above, point O is the center of the circle and OC = AC = AB. What is the value of x?

In the figure above, point O is the center of the circle and OC = AC = AB. What is the value of x?

(A) 40
(B) 36
(C) 34
(0) 32
(E) 3

This is a GMAT geometry question from the GMAT official guide for quantitative review. We get this one a lot in GMAT tutoring sessions so went ahead and made an in depth explanation. Video solution below.

This is a good one to review because it brings up an important GMAT geometry rule: the exterior angle theorem! Eh? Sounds complicated but it's actually pretty simple. Angle ACB above is an "exterior angle" of triangle AOC. The exterior angle theorem states that the exterior angle ACB equals the sum of the "remote interior angles" OAC and AOC. Basically, the exterior angle equals the sum of the angles that it's not connected to, the other two angles of the triangle known as the "remote interior angles".

Always watch out for this rule if you're dealing with triangles that share sides because: if the triangles share sides it's not unlikely that you'll have an exterior angle and will be able to use (and will probably need to use) the exterior angle theorem.

For this question we don't have much to define. We need to find the value of x.

First step in this:

  1. Draw a diagram and make easy inferences. The first one to label is that OC = AC = AB.
  2. Once you have those equal sides you can also label angles that are equal as sides that are equal within the same triangle have angles opposite them that are also equal.
  3. Now you should be thinking, hmmm, triangles sharing sides maybe there's an exterior angle. Yes, of course there is! Angle ACB. ACB is equal to AOC + OAC.
  4. Now you should also be wondering why this triangle is in a circle. How can the circle help you make an inference? For GMAT circle questions always think about "helpful radii". There's usually a radius whether it's drawn or you have to draw it that will help. In this case OA and OB are both radii so they are equal boom!
  5. That's it. You should have all angles defined by x. Go ahead and sum all of the x's  and set that equal to 180 to solve.

In the figure above, point O is the center of the circle and OC = AC = AB. What is the value of x? Solution

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