$2.$a$.$ An old pavement has layer thickness and coefficients as shown in the table below. Find the Structural Number of this pavement structure.

Due to increased traffic compared to old condition, new $SN$ requirement is $6\mathrm{.}75.$ Is the old pavement structure adequate for this increased traffic? If not, you can add another asphalt layer $($overlay$)$ on top of existing Layer $1.$ If all material properties of the new Overlay layer are the same as Layer $1,$ what should be the new Overlay thickness? Assume no damage has occurred to old pavement yet. $SN={\displaystyle \underset{?}{\overset{?}{}}}{a}_i{D}_i{m}_i$

$\backslash$table$[[$Old Pavement Layers,Layer $1,$Layer $2,$Layer $3,$Layer $4],[$Layer Type,Asphalt,Base,Subbase,ISG$],[$Layer Thickness $($inch$),5,{10}^{\text{'}\text{'}},{10}^{\text{'}\text{'}},14$