2025 AMC 12A Problem 18

Below is the professionally curated solution for Problem 18 of the 2025 AMC 12A, 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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Concepts:inequalitypermutationssymmetry

Difficulty rating: 2000

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

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.

Counting 33-subsets {a,b,c}\{a, b, c\} of {1,,8}\{1, \ldots, 8\} with ab>cab \gt c gives 3131 sets. Multiplying by 66 orderings yields 631=186.6 \cdot 31 = 186.

Thus, the correct answer is C.

Problem 18 in Other Years