2024 AMC 8 Problem 16
Below is the video solution and professionally curated solution for Problem 16 of the 2024 AMC 8, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2024 AMC 8 solutions, or check the answer key.
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Difficulty rating: 1660
16.
Minh enters the numbers through into the cells of a grid in some order. She calculates the product of the numbers in each row and column. What is the least number of rows and columns that could have a product divisible by
Video solution:
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Written solution:
There are multiples of from through . A row or column has product divisible by exactly when it contains at least one of these multiples.
Suppose rows and columns have products divisible by . Every multiple of must lie in one of those rows and also in one of those columns, or else it would create another marked row or column. Thus the multiples must fit in the intersection cells. If , then , which is too small. So at least rows and columns are needed.
This can be done by placing multiples of in a block, then placing the remaining multiples in a sixth column within two of those same rows. Then exactly rows and columns are marked, for a total of .
Thus, D is the correct answer.
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