2016 AMC 12B Problem 9

Attempt Problem 9 of the 2016 AMC 12B 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 2016 AMC 12B solutions, or check the answer key.

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

9.

Carl decided to fence in his rectangular garden. He bought 2020 fence posts, placed one on each of the four corners, and spaced out the rest evenly along the edges of the garden, leaving exactly 44 yards between neighboring posts. The longer side of his garden, including the corners, has twice as many posts as the shorter side, including the corners. What is the area, in square yards, of Carl’s garden?
256256
336336
384384
448448
512512
Answer: B
Concepts:fencepost countingsystem of equationsrectangleperimeter
Small hint:
Let the shorter side have xx posts and the longer side 2x2x posts; the four corner posts are shared
Big hint:
Total posts 2x+2(2x)−4=20;2x+2(2x)-4=20; a side with kk posts spans (k−1)⋅4(k-1)\cdot4 yards
Solution:
Let the shorter side have xx posts, so the longer side has 2x.2x. Counting all posts and subtracting the four corners counted twice, 2x+2(2x)−4=20,2x+2(2x)-4=20, giving x=4.x=4. The shorter side has 44 posts, or (4−1)⋅4=12(4-1)\cdot4=12 yards, and the longer side has 88 posts, or (8−1)⋅4=28(8-1)\cdot4=28 yards. The area is 12⋅28=336.12\cdot28=336.
Thus, the correct answer is B.
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