2022 AMC 8 Problem 23

Below is the professionally curated solution for Problem 23 of the 2022 AMC 8, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2022 AMC 8 solutions, or check the answer key.

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Concepts:caseworksymmetry

Difficulty rating: 1670

23.

A \bigtriangleup or \bigcirc is placed in each of the nine squares in a 3×33 \times 3 grid. Shown below is a sample configuration with three \bigtriangleup's in a line.

How many configurations will have three \bigtriangleup's in a line and three \bigcirc's in a line?

3939

4242

7878

8484

9696

Solution:

Let kk be the number of triangles. To have both a triangle line and a circle line, we need 3k63\le k\le6.

If k=3k=3, the three triangles must form a line. A row or column works, because the remaining six circles contain a full circle row or column. A diagonal does not work, because its complement contains no full line of circles. Thus there are 66 configurations for k=3k=3. By symmetry, there are also 66 configurations for k=6k=6.

If k=4k=4, choose the triangle line and then the extra triangle. If the line is a row or column, all 66 possible extra positions work, because one parallel row or column remains all circles. This gives 66=366\cdot6=36 configurations. If the triangle line is a diagonal, no extra position leaves a full circle line. By symmetry, k=5k=5 also gives 3636 configurations.

The total number of configurations is 6+36+36+6=846+36+36+6=84.

Thus, the correct answer is D.

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