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AkamaiBrah
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AkamaiBrah
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nice try, but having two layers of 5 is not the same as having one layer of 10. (good start but this problem is slightly -- just slightly -- more involved than the cube problem). Think logically.
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1*11*4!*4!

I consider two middle layers fully independent.
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stolyar
1*11*4!*4!

I consider two middle layers fully independent.


nope. think: what do I need to do the fill the 1st layer, then what do I need to do to fill the 2nd.
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AkamaiBrah
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1*11*4!*4!

I consider two middle layers fully independent.

nope. think: what do I need to do the fill the 1st layer, then what do I need to do to fill the 2nd.


One solution is:

assign color#1 to the top surface. There are now 11 other colors that can go on the bottom. For each of these 11 pairs, we need to fill two layers of 5 faces.

Lets fill the layer of 5 faces all adjacent to the top face. We have 10 colors to choose from so there are 10C5 ways to pick 5 colors. Since only the top and bottom were specified before, we started off with rotational symmetry, so there are 4! ways to distinctly color those 5 faces. Now we have 5 faces left and 5 colors. Since the 5 colors we just painted take away the rotation symmetry of the object, we now have 5! ways to paint the last 5 faces. So the answer is:

11 * 10C5 * 4! * 5! = 7983360

Method 2.

assign color#1 to the top surface. This time, lets color the 1st layer of 5 faces. There are 11C5 ways to pick 5 colors from 11 and 4! ways to paint them distinctly (we can rotate about the single top face). Now there are 6C5 ways to color the next 5 faces and 5! ways to make them distinct. The last face is completely determined by everything that happened before. Hence, there are 11C5 * 4! * 10C5 * 5! = 7983360.

Method 3.

Imagine the dodecahefron completely painted with color #1 being on the top face. Well, looking straight down, you can see that there are 5 ways to rotate the object keeping the color#1 face on top. Since there are 12 faces, there are 12x5 = 60 different positions for each distinct pattern.

Since there are 12 faces, there are 12! total different patterns, but every 60 of them are the same distinct pattern, so the answer is 12!/60 = 7983360.
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Alas, we all disappointed you.
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stolyar
Alas, we all disappointed you.


Nah. These are really tough. Took me years to get good at this stuff. Plus, I'm a mathematical finance guy, so I had to study this crap in school.

:)

feel better?



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