2020 AMC 12A Problem 20

Attempt Problem 20 of the 2020 AMC 12A below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2020 AMC 12A solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

20.

Let TT be the triangle in the coordinate plane with vertices (0,0),(0, 0), (4,0),(4, 0), and (0,3).(0, 3). 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.)
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Answer: A
Concepts:transformationcasework
Small hint:
Since TT is scalene, the only isometry fixing TT is the identity, so the three must compose to the identity
Big hint:
The third transformation is forced by the first two; it is allowed exactly when the product of the first two is again one of the five maps
Solution:
Because TT is a scalene right triangle, the only isometry carrying TT to itself is the identity, so a sequence works exactly when the three transformations compose to the identity.
Let rr be the 90∘90^\circ rotation and ss reflection across the xx-axis. The five allowed maps are r,r2,r3,s,r2s;r,r^2,r^3,s,r^2s; the missing nonidentity maps are the diagonal reflections rs,r3s.rs,r^3s. In an ordered triple, the third map is forced by the first two and is allowed precisely when their product is one of the five.
Exactly 55 ordered pairs have product identity: each first map is paired with its inverse. A diagonal reflection requires one rotation and one axis reflection; rr or r3r^3 may be paired on either side with ss or r2s,r^2s, giving 2⋅2⋅2=82\cdot2\cdot2=8 pairs. The remaining 25−5−8=1225-5-8=12 ordered pairs give valid sequences.
Thus, A is the correct answer.
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