Question 8540
Think of this as a rectangle inside another rectangle. The outside rectangle is 60 by 80. The inside rectangle is less by how much? If you said that each side is two time less than the width of the sidewalk, you would be right! So the inside rectangle is 60-2x by 80 - 2x. We know that if you take 1/6 of the area of the outside rectangle, it is the area of the inside rectangle. 
The outside is {{{(80)(60)}}}. The inside is {{{(60-2x)(80-2x)}}}. And the outside/6 equals the inside.
So, 
{{{((80)(60))/6=(60-2x)(80-2x)}}}
Do the math.