Answer
$\text{Length = 30 ft and Width = 15 ft}$
Work Step by Step
Let us consider that $x$=width (in feet) and $y$=length (in feet).
We are given:
$y=2x~~~(1) $
and the perimeter is:
$2x+2y=90~~~(2)$
Plug equation (1) into equation (2) to solve for $x$; then we get: $2x+(2)(2x)=90 \implies x=15$
Now, back substitute the value of $x$ into Equation (1) to solve for $y$:
$y=(2)(15) \implies y=30$
Therefore, the dimensions of the floor are: $\text{Length = 30 ft and Width = 15 ft}$