$4\mathrm{.}20$ You have been asked to design a flexible pavement, and the following daily traffic is expected for design: $5000$ single axles at $10,000lb$ each, $400$ single axles at $24,000lb$ each, $1000$ tandem axles at $30,000lb$ each, and $100$ tandem axles at $50,000lb$ each. There are three lanes in the design direction $($conservative design is to be used$).$ Reliability is $90\%,$ overall standard deviation is $0\mathrm{.}40\frac{,}{{?}_{\frac{?}{?}}}\frac{?}{\text{P}\text{S}\text{I}}$ is $1\mathrm{.}8,$ and the design life is $15$ years. The soil has a resilient modulus of $13,750l\frac{b}{i}{n}^2.$ If the TSI is $2\mathrm{.}5,$ what is the required SN$?$