$.3.($a$).$ A brass bar of $12$ mm diameter is subjected to a tensile test Observation shows that the stretch in a gauge length of $80$ mm is $0\mathrm{.}2$ mm under a pull of $30$ KN$.$ The bar diameter is found to decrease by $0\mathrm{.}0095$ mm$.$ Determine

$($i$)$ The modulus of elasticity of brass.