Answer:
x  =  80000/L   f
Step-by-step explanation:
When using only three sides for fencing a rectangular area, sides will be
"x "  and  "y "    x = 2*y    x  is the largest side of the rectangular area
                       we only fence one y side ( the other side is a river
So the perimeter is  L = x  + 2*y    or  L = 4*y
On the other hand if now the area will be 20000 f²
A = x*y = 20000    ⇒   y  =  20000/ x
As the fence material is the same as that of the maximum area
L =  4*y      L = 4* 20000 / x
L*x = 80000
Then  x  =  80000/L
Where L is the quantity of fencing material