#$4.$ The subgrade $M_r$ values for a proposed flexible pavement road are $20,000,20,000,5,000,5,000,5000,9,000,9,000,9,000,9,500,9,500,7,500$ and $20,000l\frac{b}{?}$ in ${?}^2$ for each month from January through December respectively. Determine the effective modulus of the subgrade that is equivalent to the combined effect of the different seasonal moduli.