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2019 AMC 12A Problem 16

Problem 16 of 25IntermediateNumber TheoryCounting & Probability

The numbers 1,1, 2,2, ,\ldots, 99 are randomly placed into the 99 squares of a 3×33 \times 3 grid. Each square gets one number, and each of the numbers is used once. What is the probability that the sum of the numbers in each row and each column is odd?

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Solution

There are 55 odd and 44 even numbers. Each row and column must contain an odd number of odd entries. The only way to place 55 odd entries with every row and column odd is to fill one complete row and one complete column (a plus shape of 3+31=53 + 3 - 1 = 5 cells). There are 33=93 \cdot 3 = 9 such patterns. Each pattern admits 5!5! placements of the odd numbers and 4!4! of the even numbers, so the probability is 95!4!9!=114. \dfrac{9 \cdot 5! \cdot 4!}{9!} = \dfrac{1}{14}. Thus, the correct answer is B.

More practice

Concepts: parity · basic probability · permutations

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.