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table 4x4, lines labeled 1,2,3,4, columns labeled a,b,c,d, written inside 0 or 1

Source: SRMC 2012

September 3, 2018
numbers in a tabletablecombinatorics

Problem Statement

In each cell of the table 4×44 \times 4, in which the lines are labeled with numbers 1,2,3,41,2,3,4, and columns with letters a,b,c,da,b,c,d, one number is written: 00 or 11 . Such a table is called valid if there are exactly two units in each of its rows and in each column. Determine the number of valid tables.
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