Julie's garden with perimeter of 54 ft and width of 12 ft is to have posts put around it 3 feet away from the garden's edge on all sides. They are also to be three feet apart. how many posts will she need?
Will be $15 \times 12$ yielding a $54$ foot perimeter. If we add 3 feet to all sides, we have a length of $21$ feet and a width of $18$ feet. This means we’ll need 8 posts for the length and 7 posts for the width; and then we need to deduct 4 posts since we counted the corners twice.
The answer is therefore
$8+8+7+7-4 = 26$ posts.
@Jaap Scherphuis has raised an excellent extension in the comments below. We ought to round the corners such that we minimize the perimeter.
The resulting fence shape looks like a running track, albeit more square than that. We must compute the total perimeter. We see that the shape has to have straightaway stretches of 15 feet and 12 feet for the lengths and widths respectively. At each corner, we create a quarter circle of radius 3 away from the corner. The total perimeter of this shape is therefore $54 + 6\pi$ feet. Spacing each post 3 feet apart would therefore require $18 + 2\pi \approx 24$ posts, although each post wouldn’t be exactly 3 feet apart from each other.