The area of a rectangular plot 30 feet long and 20 feet wide will be doubled by creating a border around the plot. what width should the border be to accomplish this?

The area of the plot is 600 (20x30) and it needs to be 1200 (600x2)
only the width is changed so you divide it by the length which is 30 
1200 divided 30 is 40 so the answer is 40 (another way is to just multiply the width by 2 since it's the only thing changing)

hope this helps :)