$40
The perimeter of the garden has length ...
... P = 8 m + 12 m + 8 m + 12 m = 40 m
Since there is a post every 4 m, there will be a total of ...
... (40 m)/(4 m/post) = 10 post
The cost per post is $4, so the total cost is ...
... (10 post)×($4/post) = $40
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Alternate solutions
You can consider this several ways. One way is to look at the average cost per meter. Each $4 post is good for 4 m, so the cost of posts is ...
... ($4/post)/(4 m/post) = $1/m
Then the 40 m perimeter can be expected to require $40 in posts.
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Another way to consider this is to count the posts (and/or cost) required on each side of the garden. An 8 m side will require 2 posts, so cost $8. A 12 m side will require 3 posts, so cost $12. The sum of the costs for the 4 sides is ...
... 2(8$ +$12) = $40
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Comment on fence posts
For a stand-alone fence of length 8 m, 3 posts of 4 m spacing (not 2) would be required: one at each end and one in the mddle.
Here, the sides don't stand alone. The end post of one side serves as the beginning post of the adjacent side. Hence, the total number of posts is simply the total number of 4 m sections.
