2019 AMC 10A Problem 20

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Concepts:paritybasic probability

Difficulty rating: 1820

20.

The numbers 1,2,,91,2,\dots,9 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?

121\dfrac{1}{21}

114\dfrac{1}{14}

563\dfrac{5}{63}

221\dfrac{2}{21}

17\dfrac{1}{7}

Solution:

Note that the only way to get an odd sum is if there are either 00 or 22 even numbers in the row or column.

The only way for this to happen is if the 44 even numbers form a rectangle with sides parallel to the large square.

The way to see this is we choose a spot for the first even number. Then we need to choose another square in the same row, x,x, and column, y,y, to be even.

The final even has to be in same column as xx and the same row as y.y. This forms the aforementioned rectangle.

There are four 2×22 \times 2 rectangles, two 3×23 \times 2 rectangles, two 2×32 \times 3 rectangles, and one 3×33 \times 3 rectangle.

This gives us a total of 4+2+2+1=9 4 + 2 + 2 + 1 = 9 rectangles, are arrangements for the even numbers.

There are 4!4! ways to arrange the even numbers and 5!5! ways to arrange the odd numbers.

This means that there are a total of 94!5! 9 \cdot 4! \cdot 5! configurations of squares that satisfy the condition.

There are a total of 9!9! arrangements with no restrictions. The probability is therefore 94!5!9!=114. \dfrac{9 \cdot 4! \cdot 5!}{9!} = \dfrac{1}{14}.

Thus, B is the correct answer.

Problem 20 in Other Years