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AMC 12 2020a Problem 20

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Let TT be the triangle in the coordinate plane with vertices (0,0)\left(0,0\right), (4,0)\left(4,0\right), and (0,3)\left(0,3\right). Consider the following five isometries (rigid transformations) of the plane: rotations of 90∘90^{\circ}, 180∘180^{\circ}, and 270∘270^{\circ} counterclockwise around the origin, reflection across the xx-axis, and reflection across the yy-axis. How many of the 125125 sequences of three of these transformations (not necessarily distinct) will return TT to its original position? (For example, a 180∘180^{\circ} rotation, followed by a reflection across the xx-axis, followed by a reflection across the yy-axis will return TT to its original position, but a 90∘90^{\circ} rotation, followed by a reflection across the xx-axis, followed by another reflection across the xx-axis will not return TT to its original position.)

(A) 12(B) 15(C) 17(D) 20(E) 25\textbf{(A) } 12 \textbf{(B) } 15 \textbf{(C) }17 \textbf{(D) }20 \textbf{(E) }25

Dataset text AMC 12 2000 to 2025. Quoted with source.

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