Answer
$200\ m$, $110\ m$
Work Step by Step
Step 1. Assume the width is $x\ m$, the length is then $2x-20\ m$
Step 2. The perimeter is $2(x+2x-20)=620$, we have $3x-20=310$, thus $x=110\ m$ and the length is $2(110)-20=200\ m$
Step 3. The length and width are $200\ m$ and $110\ m$ respectively.
