We have a square garden of 400 square foot.
The area of a square is:

where x: side length.
In this case:
![\begin{gathered} A=400=x^2 \\ x=\sqrt[]{400}=20\text{ ft} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/tx545uujjptkzc416w551h6lx4pmw4ygh5.png)
The perimeter of the square is the sum of the lengths of the sides of the square. As they are all equal, we can write:

The fencing is priced at $1.50 per foot. If we add the 7% sales tax to this price we get:

The fencing will be installed in all the perimeter (80 ft).
We can calculate the total cost by multiplying the sales price ($1.605 per foot) and the perimeter (80 ft):

Answer: the fencing will cost a total of $128.40