Chapter 8 - Section 8.1 - Systems of Linear Equations: Substitution and Elimination - 8.1 Assess Your Understanding - Page 556: 57

The length is $30$ ft, the width is $15$ ft.

Let x=width, y=length, in feet. Perimeter:$\qquad 2x+2y=90$ Given:$\qquad y=2x\qquad$(*) Substitute (*) into $2x+2y=90$ $2x+2(2x)=90$ $6x=90$ $x=15$ Back-substitute into (*) $y=2(15)=30$ The length is $30$ ft, the width is $15$ ft.