2020 AMC 10A Problem 23


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.)

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

Show/Hide Hints

Show/Hide Problem Tags

Problem Tags: Counting and probability Geometry

Want to contribute problems and receive full credit? Click here to add your problem!
Please report any issues to us in our Discord server
Go to previous contest problem (SHIFT + Left Arrow) Go to next contest problem (SHIFT + Right Arrow)
Category: AMC 10A
Points: 4
Back to practice