A rectangular yard 75 feet by 135 feet has a fence with a post every five feet.How many posts are needed for the entire fence?

Answer 1

perimeter = 2(75+135) = 420
number of 4 ft lengths = 420/5 = 84

so we need 84 posts

(look at the first length of 75 feet. Let's put a post at the end of each 5 ft stretch, that would take take 15 posts. Notice that the beginning of that side still has no post, but the beginning of the 135 ft side already has a post, so for this side in the same way we need 27 post, another 15 for the next side and finally 27 for the last side.
Notice that the last post filled in our missing starting point)