Answer:
Step-by-step explanation:
1) Let width = x
One side of the rectangle is covered by river. So only, 3 sides to be covered.
length = 100 - ( x + x) = 100 - 2x
Area of rectangle = length *width
= (100 - 2x)*x
f(x) = 100x - 2x^{2}

X = 96
100 137.4
137.4x=(100)(96)
137.4x=9600
137.4x/137.4=9600/137.4
x=70%
I know this is a stretch, but maybe the thickness of a nail?