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.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:divisibilityoptimizationextremal argument

Difficulty rating: 1660

16.

Minh enters the numbers 11 through 8181 into the cells of a 9×99 \times 9 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 3?3?

88

99

1010

1111

1212

Video solution:
Solution video thumbnail
Play video

Click to load, then click again to play

Written solution:

There are 2727 multiples of 33 from 11 through 8181. A row or column has product divisible by 33 exactly when it contains at least one of these multiples.

Suppose rr rows and cc columns have products divisible by 33. Every multiple of 33 must lie in one of those rr rows and also in one of those cc columns, or else it would create another marked row or column. Thus the 2727 multiples must fit in the rcrc intersection cells. If r+c10r+c\le10, then rc25rc\le25, which is too small. So at least 1111 rows and columns are needed.

This can be done by placing 2525 multiples of 33 in a 5×55\times5 block, then placing the remaining 22 multiples in a sixth column within two of those same rows. Then exactly 55 rows and 66 columns are marked, for a total of 1111.

Thus, D is the correct answer.

Problem 16 in Other Years

1985 AMC 8 · 1986 AMC 8 · 1987 AMC 8 · 1988 AMC 8 · 1989 AMC 8 · 1990 AMC 8 · 1991 AMC 8 · 1992 AMC 8 · 1993 AMC 8 · 1994 AMC 8 · 1995 AMC 8 · 1996 AMC 8 · 1997 AMC 8 · 1998 AMC 8 · 1999 AMC 8 · 2000 AMC 8 · 2001 AMC 8 · 2002 AMC 8 · 2003 AMC 8 · 2004 AMC 8 · 2005 AMC 8 · 2006 AMC 8 · 2007 AMC 8 · 2008 AMC 8 · 2009 AMC 8 · 2010 AMC 8 · 2011 AMC 8 · 2012 AMC 8 · 2013 AMC 8 · 2014 AMC 8 · 2015 AMC 8 · 2016 AMC 8 · 2017 AMC 8 · 2018 AMC 8 · 2019 AMC 8 · 2020 AMC 8 · 2022 AMC 8 · 2023 AMC 8 · 2025 AMC 8 · 2026 AMC 8