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number theory junior
Problem
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 ?
(A)
(B)
(C)
(D)
Solution
Note you can swap/rotate any configuration of rows, such that all the rows and columns that have a product of 3 are in the top left. Hence the points are bounded by a rectangle. This has area and rows and columns divisible by . We want and minimized. If , we achieve minimum with . If ,our best is . Note if , then , and hence there is no smaller answer, and we get .
Final answer
D