2025 AMC 12A Problem 18

Attempt Problem 18 of the 2025 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 2025 AMC 12A solutions, or check the answer key.

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18.

How many ordered triples (x,y,z)(x, y, z) of distinct nonnegative integers less than or equal to 88 satisfy xy>z,xy \gt z, zx>y,zx \gt y, and yz>x?yz \gt x?

3636

8484

186186

336336

486486

Answer: C
Concepts:inequalitypermutationssymmetry
Difficulty rating: 2000
Solution:

If any variable is 0,0, say z=0,z = 0, then zx=0>yzx = 0 \gt y is impossible. So x,y,z{1,,8}x, y, z \in \{1, \ldots, 8\} are distinct positive integers.

The conditions are symmetric. For distinct values a<b<c,a \lt b \lt c, we have ac>bac \gt b and bc>abc \gt a automatically, so the only real constraint is ab>c.ab \gt c. When it holds, all 66 orderings work.

For each possible smallest value a,a, the numbers of pairs a<b<c8a\lt b\lt c\le8 satisfying ab>cab\gt c are a123456count01110631. \begin{array}{c|rrrrrr} a&1&2&3&4&5&6\\ \hline \text{count}&0&11&10&6&3&1. \end{array} For fixed a,b,a,b, this count comes from choosing cc between b+1b+1 and min(8,ab1).\min(8,ab-1). Thus there are 3131 unordered triples, and all 66 orderings of each work. The answer is 631=186.6\cdot31=186.

Thus, the correct answer is C.

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