2023 AMC 10A Problem 20

Attempt Problem 20 of the 2023 AMC 10A 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 2023 AMC 10A solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

20.

Each square in a 3×33 \times 3 grid of squares is colored red, white, blue, or green so that every 2×22 \times 2 square contains one square of each color. One such coloring is shown on the right below. How many different colorings are possible?
2424
4848
6060
7272
9696
Answer: D
Concepts:arrangements with restrictionscasework
Small hint:
Label the cells row by row as a,a, b,b, c;c; d,d, e,e, f;f; g,g, h,h, i;i; the top-left four are a permutation of all four colors
Big hint:
Then {c,f}\{c, f\} and {g,h}\{g, h\} are each the two remaining colors in some order, and ii is forced but works only when f≠hf \ne h
Solution:
Label the cells row by row as a,a, b,b, c;c; d,d, e,e, f;f; g,g, h,h, i.i. The top-left block a,b,d,ea, b, d, e is a permutation of the four colors, so 4!=244! = 24 ways. The block {b,c,e,f}\{b, c, e, f\} is also all four colors, and b,eb, e are fixed, so {c,f}\{c, f\} is the remaining two in some order: 22 ways. Same story for {g,h},\{g, h\}, the two colors apart from d,e,d, e, another 22 ways. That leaves i,i, forced to whatever color is missing from {e,f,h},\{e, f, h\}, and that only works when f≠h.f \ne h. Of the 2⋅2=42 \cdot 2 = 4 order combinations, exactly one has f=h,f = h, so 33 survive. The total is 24⋅3=72.24 \cdot 3 = 72. Therefore, the answer is D.
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