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Combinatorics Difficulty 4.1 AIME Prove it Japan

A 3×43 \times 4 grid is given. Answer the number of possible cases to write a number among 11, 22, 33 or 44 in each square so that
* no number appears twice (or more) in the same row, and
* no number appears twice (or more) in the same column.

Solution

Denote by aija_{ij} the entry that lies in the ii-th column and the jj-th row. The given conditions yields that each 11, 22, 33 and 44 appear exactly once in each column.
We may assume without loss of generality that a1j=ja_{1j} = j for j=1,2,3,4j = 1, 2, 3, 4. We would multiply the answer by 4!4! afterwards. We may also assume that a22=a33=1a_{22} = a_{33} = 1. We would multiply the answer by 3×23 \times 2 afterwards.
If a21,a314a_{21}, a_{31} \neq 4, then a23=a32=4a_{23} = a_{32} = 4. We can choose which of a24a_{24} and a34a_{34} to be 22, and the rest entries would be determined uniquely. So we have two ways.
Otherwise, if a21=4a_{21} = 4, there are only one way to fill the other squares. The case a31=4a_{31} = 4 is similar. So we have two ways.
Therefore, the answer is 4!×3×2×(2+2)=5764! \times 3 \times 2 \times (2+2) = 576.

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