assume that the edge of a square is 1 unit add squares to this shape so that it has a perimeter of 18

That would be a little hard, since the perimeter of the square is the the sum of the four sides.

18/4 = 4.5

To solve this problem, we need to find out how many squares we need to add to the original square so that the new shape has a perimeter of 18 units.

Let's start by analyzing the original square:

- Each side of the original square has a length of 1 unit.
- The perimeter of the original square is given by the sum of all four sides, which is 1 + 1 + 1 + 1 = 4 units.

Since the original square has a perimeter of 4 units, we need to add squares in such a way that the additional perimeter they contribute sums up to 18 - 4 = 14 units.

To calculate how many additional squares we need, we should consider that:
- Each additional square adds 4 units to the overall perimeter (since each side of the square has length 1 unit).
- Therefore, we can divide the additional perimeter (14 units) by the amount of perimeter contributed by each additional square (4 units) to find the number of squares needed.

Using this reasoning, we can calculate the number of additional squares needed as follows:

Number of Squares = Additional Perimeter / Perimeter per Square
= 14 units / 4 units
= 3.5

Since the result is not a whole number, we need to round it up to the nearest whole number because we cannot have a fractional amount of squares. Therefore, we need to add 4 squares to the original square to achieve a new shape with a perimeter of 18 units.

So, the final shape will consist of a central square with a side length of 1 unit, surrounded by 4 additional squares with side lengths of 1 unit each.