2022 AMC 12A Problem 7

Attempt Problem 7 of the 2022 AMC 12A below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2022 AMC 12A solutions, or check the answer key.

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7.

A rectangle is partitioned into 55 regions as shown. Each region is to be painted a solid color - red, orange, yellow, blue, or green - so that regions that touch are painted different colors, and colors can be used more than once. How many different colorings are possible?

120120

270270

360360

540540

720720

Answer: D
Concepts:graph theorymultiplication principle
Difficulty rating: 1380
Solution:

The bottom-middle region shares a border with all four other regions. Color it first in 55 ways.

The top-left region borders it, giving 44 choices. Each of the three remaining regions borders exactly two already-colored regions, which have different colors, leaving 33 choices apiece.

The total is 54333=540.5\cdot4\cdot3\cdot3\cdot3=540.

Thus, the correct answer is D.

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