The first thing you should do for this case is to find the perimeter of the rectangle.
We have then:
P = 2L + 2W
Where,
L: long
W: wide.
Substituting the values:
P = 2 * (20) + 2 * (8)
P = 56 m
Then, to find the number of posts we have:
N = P / d
where,
P: perimeter
d: distance between posts.
Substituting:
N = (56) / (4) = 14
The total cost will be:
(5.50) * (14) = $ 77
Answer:
the total cost for all the posts wil be 77 $