You can ignore the diagram completely (as I did when I solved it) because the question tells you that you get a figure with "2 pyramids each with a square base that they share". Imagine what one pyramid looks like - a square with triangles attached to each side and the tips of the triangles stuck together. Imagine how would you have two pyramids with a common base. There would be 4 triangles on the lower side of the square too and their tips would be stuck together. So in all, the figure would have 8 faces (4 triangles + 4 triangles) and 12 edges ( 4 edges where the top triangles join, 4 edges where the bottom triangles join and 4 edges of the square).
All in all you will have 8 + 12 = 20 faces + edges
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Dear vikasbansal227
I'm happy to respond. This is a great question.

First of all, here is a labeled version of the polyhedral net you provided:

Incidentally, in your version, you indicate a solid line from B to I, but this should be a dashed line.

When this is folded up, the name of the resultant shape is the Octahedron, one of the five Platonic Solids. It good practice for 3D thinking to study the five Platonic Solids. But, suppose you didn't know about the Octahedron: how would we answer the question?

Notice, that when we fold up, A & C come together, so we could say that B is the vertex of one pyramid, with four edges going down to D, I, J, and A/C. These latter four points form a square that is the "equator" of the shape, so there are four edges around this square. Then, we fold F & H together, forming the second pyramid. This second pyramid has G as its vertex, with four edges come down to the points E, D, I, and F/H; these latter four points also form a square. Then, segment DI acts as a hinge, and along this hinge we fold one pyramid down to meet the other, so that the square bases of the two pyramids meet and become one. Point A/C joins with point E, and point J joins with point F/H.
The final shape has a lower downward-pointing pyramid, with point B as the lower vertex. Four edges go up from this vertex to the square "equator." This square "equator" consists of points D + I + A/C/E + F/H/J, and like any square, it contains four edge segments. Finally, the upper upward-pointing pyramid has vertex G, and four edges go from this vertex down to the four vertices of the square "equator." That's a total of 12 edges.

We can simply count the 8 faces while it is still flat.

E + F = 12 + 8 = 20 ==> OA = (C)

The thing that is a little mind-blowing about the octahedron is this. In the above description, I was discussing the "upper vertex," the "lower vertex," and the "square equator," but because the shape is 100% symmetrical, if we simply turn the shape, any vertex can be the tip of the upper pyramid. There are actually three interlocking squares formed by different combinations of the vertices. This Wikipedia page has a gif of a rotating octahedron, which may help you visualize it more.

Does all this make sense?
Mike
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[quote="mikemcgarry"][quote="vikasbansal227"]
"Incidentally, in your version, you indicate a solid line from B to I, but this should be a dashed line." Thanks for you post Mike. Just as a reference though, In the OG2015, there is a solid line from B to I

Hello Mike,
I went through wikipedia link you have mentioned and got to know the shape of octahedron
But i could not interpret the shape from original diagram given in the question

Dear divya517,
My friend, this is a HARD question. If the student has run across the octahedron ahead of time, then that probably would give an advantage on this question. The octahedron is a very hard shape to understand, and getting it just from the diagram and the brief verbal description is extremely challenging. If you find 3D Geometry particularly challenging, it might be worthwhile to investigate this a little further to stretch yourself. The octahedron is one of a set of 3D figures known as the "Platonic Solids." The cube is another, and there are five altogether. Investigate these, as well as prisms and pyramids in general. Learning to think in 3D about abstract geometry is a skill that deepen other categories of mathematical thinking.
Does all this make sense?
Mike
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Hey Mike, I have a general question for problems like this. If I see a figure like this and it's extremely complex to visualize or understand the folds, will the question stem usually tell you the end shape after all the folding? I'm no GMAT expert by any means, but I solved this in under a minute because I basically looked at the figure and thought "to heck with that, can't understand what it's telling me", so I kept reading and the question actually tells me what the final shape is: Two pyramids that share the square face. There's only one way for two pyramids to share the same square face. Everything else was easy, didn't use the figure at all. So again my question to you is, will my method work for other problems similar to this? I want to generalize this so that I can put this in my "pattern recognition" notebook.

Dear DarthestVader,
I'm happy to respond. May the Force be with you, my friend.

You know, someone could sit for 100 days in a row, taking a GMAT each day, and might well never see a question anything like this. This is an exceedingly rare topic. This entire discussion concerns something that might about as likely as seeing a unicorn crossing a rainbow!

As a general rule, even hard GMAT math questions are designed to be solved with a certain amount of elegance, and certainly thinking about the figure in this problem just as the two pyramids joined at the bases is a kind of elegant solution, so in the exceedingly rare case that such a question reappeared, I would consider it likely that the text would include some kind of description of the final shape. That way, most folks who have the intelligence to excel in B-school and in the business world could answer the question.

Remember that even with a hard question, the GMAT is trying to discriminate between high scoring students and low scoring students, those for whom B-school will be easy vs. those for whom it will be a challenge. If the test writers just gave the polygonal net with no description of the output shape, then really only folks who had a very strong background in math would get it correct, and 99% of the test-takers would get it wrong. That's not the kind of discrimination that the test is trying to do, separating math geniuses from everyone else. From the perspective of the psychodynamics of testing, such a question would be an abysmal failure, because it simply doesn't select for what the test maker really wants to know.

Remember that the test writers have definite and specific goals in creating this test. The Quant section is a section to select those who will excel in B-school and in the business world. It is not designed to evaluate people pursing doctoral work in Mathematics. Part of being successful on the GMAT is appreciating the test makers' priorities.

Does all this make sense?
Mike
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Thanks for your detailed response Mike! May the force be with you!

thank you Karishma.. was confused by reading the Magoosh's post ..

This link will help to clear any doubt,if any. http://www.dkfindout.com/uk/maths/geome ... d-pyramid/

Responding to a pm:


The two pyramids share the square. So the square is hidden inside them. Look at this figure.

The colourful figure inside shows two pyramids joined together.
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When 4 triangles are joined together to make the top figure, we get 4 edges.

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Thank you very much Karishma..This is easier solution and time saving Approach.

Does anyone else only count 6 edges? I know 14 is not an answer choice, but counting 12 edges seems to double count in my mind? Like the base of the two pyramids there are 4 edges that both pyramids share, then each of the pyramids has a top edge? Why do we count the base edges twice?

I just ignored the diagram closed my eyes and started to count

it was sooo easy

maybe it will work for someone else as well
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What do you mean "top edge"?

The pyramid at the top has 4 edges and the one at the bottom has 4 edges. Looks at the diagram here:
https://gmatclub.com/forum/when-the-fig ... l#p1787756
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You are confusing 'edges' with 'vertices'.

Face: is the flat surface of a solid figure. A face of a solid figure can be a square, a rectangle, a triangle or a circle.
Edge: is the line segment two faces of a solid meet.
Corner (vertex): is a corner where three or more edges of a solid meet.

I'm still trying to understand the bolded part. Isn't it just one edge where the four triangle faces join? They join at one spot after all.

At first I count 12 edges and 9 faces. But they dont count the square base.

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