2019 AMC 10A Problem 20
Below is the professionally curated solution for Problem 20 of the 2019 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 10A solutions, or check the answer key.
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Difficulty rating: 1820
20.
The numbers are randomly placed into the squares of a 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?
Solution:
Note that the only way to get an odd sum is if there are either or even numbers in the row or column.
The only way for this to happen is if the 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, and column, to be even.
The final even has to be in same column as and the same row as This forms the aforementioned rectangle.
There are four rectangles, two rectangles, two rectangles, and one rectangle.
This gives us a total of rectangles, are arrangements for the even numbers.
There are ways to arrange the even numbers and ways to arrange the odd numbers.
This means that there are a total of configurations of squares that satisfy the condition.
There are a total of arrangements with no restrictions. The probability is therefore
Thus, B is the correct answer.
Problem 20 in Other Years
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