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A country is divided into exactly 4 regions. Miki is sketching a map of the country's regions and plans to choose a color scheme for her map in which each region will be exactly 1 of 3 different colors and no adjacent regions will be the same color. One possible color scheme is for Regions 1,3 and 4 to have the same color and Region 2 to have a different color. How many color schemes are possible for Mikis map?

A) 6
B) 8
C) 9
D) 24
E) 36

We know that Miki has three available colors, they can color the map using the following logic

  • Paint Region 1 in 3 ways ⇢ No restriction
  • Paint Region 2 in 2 ways ⇢ Any color except the one used in Region 1
  • Paint Region 3 in 2 ways ⇢ Any color except the one used in Region 2
  • Paint Region 4 in 2 ways ⇢ Any color except the one used in Region 2

Total = 3 * 2 * 2 * 2 = 24

Option D
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Clearly, 2 is adjacent to all and 1,3,4 are completely independent

so pick color of 2 and then assign colors to rest
3x{2 +3*2] = 3x8=24
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gmatophobia why are we not rearranging it in 4!/3! ways?
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Total way to arrange in 4 regions = 4!
Ways to arrange with the 2 being adjacent = 2!(4-2+1)!

Total color schemes: 4! - 2!(4-2+1)! = 12 x 2 ways = 24
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­This is a problem of graphs

1
|
|
2 ---- 4
|
|
3


2 is adjoint to 3 three but the rest 3 are not adjacent to each other

so if there are 3 choices for 2 then each regions will have 2 options to choose from

3 * 2 * 2 * 2 = 24 options­
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This is more like a logic puzzle than a math problem.

In general, I think it is best to approach these types of problems by picking a region and listing all the possible colors that could be assigned to it. In this case, region 1. If no other colors have been drawn on the map, region 1 can be colored any of the 3 colors.

If you move to region 2, since it borders region 1 and region 1 has already been assigned a color, region 2 can be either of the 2 colors not assigned to region 1.

Similarly, in region 3, as it borders region 2, it can be colored either of the 2 colors that region 2 is not.

And finally, as region 4 borders region 2, it can be either of the 2 colors that region 2 is not.

3x2x2x2 = 24.
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Quote:
A country is divided into exactly 4 regions. Miki is sketching a map of the country's regions and plans to choose a color scheme for her map in which each region will be exactly 1 of 3 different colors and no adjacent regions will be the same color. One possible color scheme is for Regions 1,3 and 4 to have the same color and Region 2 to have a different color. How many color schemes are possible for Mikis map?

A) 6
B) 8
C) 9
D) 24
E) 36

I tried to do it by counting cases. The main point to remember is that 2 can never have the same colour as 1, 3, or 4.

Colour 1Colour 2Colour 3
Case 1:1, 3, 42-
Case 2:1, 324
Case 3:1, 423
Case 4:123, 4

Now, each case can arrange themselves in 3! ways i.e. 6 ways.

4 cases x 6 ways = 24.

Answer D.
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