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I got 21 by drawing it out (in under 2 minutes), but that's not a very scientific approach (which doesn't mean it's a bad approach; you get graph lined scratch paper on the GMAT, so if you can visualize the shape it's a good way to tackle the question).harsh.champ wrote:Each side of a given polygon is parallel to either the X or the Y axis. A corner of such a polygon is said to be convex if the internal angle is 90o or concave if the internal angle is 270o. If the number of convex corners in such a polygon is 25, the number of concave corners must be:
(1)20
(2)0
(3)21
(4)22
(5)23
The angles in an n-sided polygon add to (n-2)*180. If we have 25 angles measuring 90 degrees, and x angles measuring 270 degrees, then we have 25+x sides in total, soharsh.champ wrote:Each side of a given polygon is parallel to either the X or the Y axis. A corner of such a polygon is said to be convex if the internal angle is 90o or concave if the internal angle is 270o. If the number of convex corners in such a polygon is 25, the number of concave corners must be:
(1)20
(2)0
(3)21
(4)22
(5)23
Yes, that will be true in general; in an n-sided shape, the sum of our angles is 180n - 360. If we have only 'x' 90 degree angles and 'y' 270 degree angles in our shape, we have x+y angles in total, and we need 90x + 270y to be equal to 180x + 180y - 360. So you might be able to see immediately that we need x to be 4 more than y to account for the '-360' part; or we can prove this algebraically:Stuart Kovinsky wrote: If you draw out some simpler shapes, you can quickly see that convex - concave will always equal 4. I'm sure that someone has a beautiful proof for this deduction, but it's certainly not one you'd ever need to know for the GMAT.
_____________Stuart Kovinsky wrote:I got 21 by drawing it out (in under 2 minutes), but that's not a very scientific approach (which doesn't mean it's a bad approach; you get graph lined scratch paper on the GMAT, so if you can visualize the shape it's a good way to tackle the question).harsh.champ wrote: (5)23
If you draw out some simpler shapes, you can quickly see that convex - concave will always equal 4. I'm sure that someone has a beautiful proof for this deduction, but it's certainly not one you'd ever need to know for the GMAT.
What's the source of this question?
If you're preparing for the GMAT, you should focus on the types of questions that are going to show up on the actual exam. If your tutor is a math expert but not familiar with the GMAT, then he or she isn't a good person to be giving you questions.harsh.champ wrote:
_____________
This question is a part of the test given by a tutor at my native place.
Actually,now I am a bit confused as to what concepts are not necessary for GMAT preparation and what concepts are indispensable.I thought this was supposed to be a good geometry problem.
Please can you throw some light if I am doing some mistake in my preparation strategy!!!
_____________________Stuart Kovinsky wrote:If you're preparing for the GMAT, you should focus on the types of questions that are going to show up on the actual exam. If your tutor is a math expert but not familiar with the GMAT, then he or she isn't a good person to be giving you questions.harsh.champ wrote:
_____________
This question is a part of the test given by a tutor at my native place.
Actually,now I am a bit confused as to what concepts are not necessary for GMAT preparation and what concepts are indispensable.I thought this was supposed to be a good geometry problem.
Please can you throw some light if I am doing some mistake in my preparation strategy!!!
While it's true that working on challenging questions will help you improve, those questions need to be relevant to the GMAT. Most test takers only see 3-5 geometry questions in total and those questions focus primarily on basic geometry. Of course if you're doing great on the GMAT you'll see more advanced applications of those basic principles, but it's the basic principles you need to master.
If you don't already have it, buy the Official Guide and review the preview math chapters to get a feel for the level of math on which you'll be tested.
If you're comfortable with the math in general, you should start with intermediate questions (the ones you'll see at the beginning of the test) and then progress to advanced questions. One common mistake that people make is to ignore the intermediate questions; a common result of this is that they're baffled by the first few questions in each section and never get to the advanced questions for which they're prepared.harsh.champ wrote: _____________________
Thanks for the advice.
I have the OG 12th edition and i use it from time to time.
Actually,the tutor's mentality is to prepare us even for tough questions.Though most questions tested on the GMAT are of a far easier variety but that puts us in a comfortable state having solved the advanced ones.
As on the GMAT,you can't skip any question,he said that you should have the level of preparation a notch higher than the question level that you will expect on the D-Day.
I have reviewed the math chapters of the OG and have got a cetain feel about the kind of questions featured.
If my basics are strong,is it Ok to skip that section and move to my weaker sections or should I maintain equivalent level of efforts in all the sections??Right now,I am pursuing the latter.
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