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Given 5 colors to choose from, how many ways can we color the four unit squares of a 2×2 board, given that two colorings are considered the same if one is a rotation of the other? A color can be used more than once

 Dec 14, 2018
 #1
avatar+776 
0

We have 5 options and must choose 4. However, the same color can be used more than once. 5*4 = 20, and  (204) = 4845.

 

We did overcount (Example: 1234 is the same as 2341, but we counted it twice), so to take away overcounting, we divide 4845 by 5 to get 969. With some other casework and steps shown below, you will end up with 165.

 

Do not try to list out the answers because there are many and that is inefficient. Always use number theory. 

 

- PM

 Dec 15, 2018
edited by PartialMathematician  Dec 15, 2018
 #2
avatar+776 
0

I am not 100% sure about my answer though.

PartialMathematician  Dec 15, 2018
 #3
avatar+776 
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You could also use Burnside's lemma.

PartialMathematician  Dec 15, 2018
 #6
avatar+118703 
+1

PM:  It may be a form of flattery but it is not very polite to change your answer to match mine.   wink

Melody  Dec 15, 2018
 #7
avatar+776 
+1

Ok, sorry about that. blush

PartialMathematician  Dec 16, 2018
 #4
avatar+118703 
+3

All the same colour = 5 ways

3 the same and one different = 5C2*2 = 20 ways

2 of one colour and 2 of another = 5C2*2 =20ways

2 the same and 2 different = 5C3*3*3 = 90ways

all colours different = 5C4*3*2= 30 ways

 

5+30+20+20+90 = 165 ways

 Dec 15, 2018

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