1992 AMC 8 Problem 22

Attempt Problem 22 of the 1992 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 1992 AMC 8 solutions, or check the answer key.

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

22.

Eight 1×11 \times 1 square tiles are arranged as shown so their outside edges form a polygon with a perimeter of 1414 units. Two additional tiles of the same size are added to the figure so that at least one side of each added tile is shared with a side of one of the squares in the original figure. Which of the following could be the perimeter of the new figure?
1515
1717
1818
1919
2020
Answer: C
Concepts:perimetercasework
Small hint:
Adding a tile that shares exactly one side changes the perimeter by +2+2; sharing two sides changes it by 00
Big hint:
With two added tiles, the total change can be 0,0, 2,2, or 44; add these to 1414
Solution:
A tile that shares exactly one side adds 22 to the perimeter (four new edges minus two hidden), while a tile sharing two sides adds 0.0.
With two added tiles the perimeter can change by 0,0, 2,2, or 4,4, giving new perimeters of 14,14, 16,16, or 18.18. Of the choices, only 1818 is possible.
Thus, the correct answer is C .
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