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I have a $2\times3$ grid which is colored with four colours. How many colourings do there exists such that every two adjacent squares have different colours? I don't know how to start.

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  • $\begingroup$ Start by trying to solve similar problems for smaller grids and fewer colors. $\endgroup$ – MJD Jan 14 '17 at 18:39
  • $\begingroup$ @MJD What is the basic idea? $\endgroup$ – Problemsolving Jan 14 '17 at 18:45
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    $\begingroup$ This idea should work but not in general: Top middle cell has 4 choices, neighbors have 3 choices, then take cases for the last two cells. $\endgroup$ – greenturtle3141 Jan 14 '17 at 18:47
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Color the grid by columns (top and bottom cells) going from left to right:

You have $4\cdot 3=12$ ways to color the $1$st column. Now given that you've made your choice of colors for the $1$st column, the number of ways to color the $2$nd column will drop down from $12$ to $7$. The same happens for the 3rd column, once you've colored the $2$nd one . So the total number is $$12\cdot 7 \cdot 7=588.$$

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  • $\begingroup$ WHy 7? They are 6? $\endgroup$ – Problemsolving Jan 14 '17 at 20:15
  • $\begingroup$ Let A, B,C, D be the four colors. Suppose the $1$st column is (A,B) (A on the top, and B on the bottom). Can you list the possibilities for the $2$nd column? $\endgroup$ – MathChat Jan 14 '17 at 20:23
  • $\begingroup$ Sorry, you have right! $\endgroup$ – Problemsolving Jan 14 '17 at 20:34

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