Answer
Tthe length of the rectangle is $10 \ ft$ and the width is $\frac{1}{2} \cdot 10=5 \ ft$
Work Step by Step
Assume that the length of the rectangle is $x$; then its width will be $\frac{1}{2}x$, now we have $$Perimeter \ =2x+2\cdot \frac{1}{2}x =30\ ft$$ That is $$3x=30 \ ft \Longrightarrow x=10 \ ft$$ Hence the length of the rectangle is $10 \ ft$ and the width is $\frac{1}{2} \cdot 10=5 \ ft$
