#### Answer

The area of this rectangle is $25 ft^2$.

#### Work Step by Step

The perimeter of a square is given by the formula:
$P = 4s$, where $s$ is the measure of one side of the square.
The area of a square is given by the formula:
$A = s^2$, where $s$ is the measure of one side of the square.
In order to find the area of the square, we need to find the measure of one of its sides.
Let us use what we know. We actually know that the square has a perimeter of $20$ ft., so we plug in this information into the formula for the perimeter of a square and solve for $s$:
$20 = 4s$
Divide both sides of the equation by $4$ to solve for $s$:
$s = 5$
Now, we can plug in $5$ for $s$ in the formula for the area of a square:
$A = 5^2$
Evaluate the exponent to solve for $A$:
$A = 25$
The area of this rectangle is $25 ft^2$.