Suppose the rest area's length be x meters.
Perimeter = 2(length + widths)
So width = 120/2 - x = 60-x meter.
Area of the rest area A(x) = x(60-x) = -x2+60x = 302 -[x2-2x(30) +302] = 900 -(x-30)2
[ Refer to section where you complete square to find min/max value of a quadratic eqn]
Maximum value of the quadratic A(x) is 900

Soumendra M.
05/31/19