2016 AMC 10B Problem 11

Let $l$ be the number of posts on a long side, including corners, and let $s$ be the number of posts on a short side, including corners. The total number of posts is $$2l+2s-4=20,$$ where the $-4$ avoids double-counting the corners. Since a long side has twice as many posts as a short side, $l=2s$. Thus $2(2s)+2s-4=20$, so $s=4$ and $l=8$. There are $3$ equal gaps on a short side and $7$ equal gaps on a long side. With $4$ yards between neighboring posts, the side lengths are $12$ and $28$, so the area is $12\cdot28=336$. Thus, the correct answer is B.