What is the ratio of the numerical value of the area, in square units, of an equilateral triangle of side length 18 units to the numerical value of its perimeter, in units? Express your answer as a common fraction in simplest radical form.
Perimeter = 18 + 18 + 18 = 54
area = sqrt3 /4 * 18^2 =81 sqrt3
81 sqrt3 / 54 = 3/2 * sqrt 3