Question 659113
The problem states that if the width was increased by 20, that would equal the length. That gives us information we can use to write expressions to represent the width and length.

Call the width w.
Length must then be w+20

Using your formula for perimeter:
p=2l + 2w
200=2(w+20) + 2w
200=2w+40+2w
200=4w+40
200-40=4w+40-40
160=4w+0
160=4w
1/4*160=4w*1/4
40=w

If the width is 40 feet, the length must then be 40+20 or 60 ft. Check to see if that gives you the perimeter of 200 feet and you will know it is a correct solution.