2012 AMC 12A Problem 15


A 3×33\times3 square is partitioned into 99 unit squares. Each unit square is painted either white or black with each color being equally likely, chosen independently and at random. The square is then rotated 9090^\circ clockwise about its center, and every white square in a position formerly occupied by a black square is painted black. The colors of all other squares are left unchanged. What is the probability that the grid is now entirely black?

(A) 49512(B) 764(C) 1211024(D) 81512(E) 932\textbf{(A)}\ \dfrac{49}{512}\qquad\textbf{(B)}\ \dfrac{7}{64}\qquad\textbf{(C)}\ \dfrac{121}{1024}\qquad\textbf{(D)}\ \dfrac{81}{512}\qquad\textbf{(E)}\ \dfrac{9}{32}


Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.

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Category: AMC 12A
Points: 3
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