1988 AMC 8 Problem 24

Attempt Problem 24 of the 1988 AMC 8 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 1988 AMC 8 solutions, or check the answer key.

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

24.

The square in the first diagram “rolls” clockwise around the fixed regular hexagon until it reaches the bottom. In which position will the solid triangle be in diagram 4?4?
Answer: A
Concepts:transformationregular polygon
Small hint:
Each time the square rolls over one edge of the hexagon it pivots about a shared corner and turns 150∘150^\circ clockwise
Big hint:
Going from the top edge to the bottom edge is 33 rolls, a total of 3×150∘=450∘,3 \times 150^\circ = 450^\circ, which is one full turn plus an extra quarter turn clockwise
Solution:
Each roll of the square over one edge of the hexagon turns it 150∘150^\circ clockwise. (At the pivot corner the square turns through 360∘360^\circ minus the square’s own 90∘90^\circ corner minus the hexagon’s 120∘120^\circ interior angle, which leaves 150∘.150^\circ.)
From the top of the hexagon to the bottom is 33 rolls, for a total of 3×150∘=450∘.3 \times 150^\circ = 450^\circ. That is one complete revolution (360∘360^\circ) plus an extra 90∘90^\circ clockwise.
The triangle starts pointing straight up, so after the extra quarter turn clockwise it points to the right.
Thus, the correct answer is A .
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