2011 AMC 8 Problem 3

Attempt Problem 3 of the 2011 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 2011 AMC 8 solutions, or check the answer key.

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

3.

Extend the square pattern of 88 shaded and 1717 unshaded square tiles by attaching a border of shaded tiles around the square. What is the ratio of shaded tiles to unshaded tiles in the extended pattern?
8:178:17
25:4925:49
36:2536:25
32:1732:17
36:1736:17
Answer: D
Concepts:ratio and proportionbasic counting
Small hint:
A border around a 55 by 55 square adds the outside ring of tiles.
Big hint:
The original unshaded tiles stay the same.
Solution:
The original pattern is a 5×55\times5 square. Extending it by one tile on every side makes a 7×77\times7 square, so the new shaded border contains 72−52=247^2-5^2=24 tiles. Together with the original 88 shaded tiles, there are 24+8=3224+8=32 shaded tiles. The original 1717 unshaded tiles do not change, so the ratio of shaded tiles to unshaded tiles is 32:17.32:17.
Thus, D is the correct answer.
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