$7\mathrm{.}12$ A strip of elastomer $1mm1mm10cm$ that obeys the affine network model is stretched to $40cm$ length at $T=300K$ at the engineering stress $1{10}^{7}Pa.$ The volume occupied by each monomer is $b^3=80{}^3.$

$($i$)$ What is the relative elongation $?$

$($ii$)$ Find the shear modulus $G.$

$($iii$)$ How many moles of network strands are in this rubber strip?

$($iv$)$ Estimate the number of monomers between crosslinks.