Tom is going to build a fence around his garden which is a rectangle measuring 8 m by 20 m. he will first put in posts which will be 4 m apart. if the posts cost $5.50 each, what will be the total cost for all the posts?

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
 
 the total cost for all the posts wil be 77 $