Answer :
There are 7 posts along one 8-yard side and 5 posts along one 6-yard side. It means that he used:
2 * 7 + 2 * 5 + 4 (posts on the corners) = 14 + 10 + 4 = 28 posts
2 * 7 + 2 * 5 + 4 (posts on the corners) = 14 + 10 + 4 = 28 posts