AMC 10 Step by Step
2020 AMC 10AProblem 23P21-25~8 minPrint

Let TT be the triangle in the coordinate plane with vertices (0,0),(4,0),(0,0), (4,0), and (0,3).(0,3). Consider the following five isometries (rigid transformations) of the plane: rotations of 90,180,90^{\circ}, 180^{\circ}, and 270270^{\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 180180^{\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 9090^{\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.)

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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site.