2010 AMC 12B Problem 17

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17.

The entries in a 3×33\times3 array include all the digits from 11 through 9,9, arranged so that the entries in every row and column are in increasing order. How many such arrays are there?

1818

2424

3636

4242

6060

Answer: D
Concepts:arrangements with restrictionscasework
Difficulty rating: 1980
Solution:

Write aija_{ij} for the entry in row i,i, column j.j. The conditions force a11=1,a_{11}=1, a33=9,a_{33}=9, and a22{4,5,6}.a_{22}\in\{4,5,6\}.

If a22=4,a_{22}=4, then {a12,a21}={2,3}\{a_{12},a_{21}\}=\{2,3\} and {5,6,7,8}\{5,6,7,8\} split as complementary pairs filling the rest of the last row and column: (42)=6\binom42=6 splits times 22 orders for {2,3}\{2,3\} gives 1212 arrays. By symmetry a22=6a_{22}=6 also gives 12.12.

If a22=5,a_{22}=5, then {a12,a13,a23}\{a_{12},a_{13},a_{23}\} and {a21,a31,a32}\{a_{21},a_{31},a_{32}\} are complementary subsets of {2,3,4,6,7,8}\{2,3,4,6,7,8\} subject to the ordering constraints. The first set can be any three-element subset except {2,3,4}\{2,3,4\} or {6,7,8},\{6,7,8\}, giving (63)2=18\binom63-2=18 arrays.

Altogether 12+12+18=42.12+12+18=42.

Thus, the correct answer is D.

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