2016 AMC 12B Problem 9

Below is the professionally curated solution for Problem 9 of the 2016 AMC 12B, 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 of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:fencepost countingsystem of equationsrectangleperimeter

Difficulty rating: 1440

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

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 (41)4=12(4-1)\cdot4=12 yards, and the longer side has 88 posts, or (81)4=28(8-1)\cdot4=28 yards. The area is 1228=336.12\cdot28=336.

Thus, the correct answer is B.

Problem 9 in Other Years