$Stress\left( {\frac{F}{A}} \right) = 5 \times {10^7}N{m^{ - 2}}$

$\Delta V = 0.02\%  = 2 \times {10^{ - 4}}{m^3}$

$\frac{{\Delta r}}{r} = ?$

$\gamma  = \frac{{stress}}{{strain}} \Rightarrow strain\left( {\frac{{\Delta \ell }}{{{\ell _0}}}} \right) = \frac{\gamma }{{stress}}\,\,...\left( i \right)$

$\Delta V = 2\pi {\ell _0}\Delta r - \pi {r^2}\Delta \ell $                       $...\left( {ii} \right)$

From eqns $(i)$ and $(ii)$ putting the value of 

$\Delta \ell ,{\ell _0}\,and\,\Delta V\,and\,solving\,we\,get$

$\frac{{\Delta r}}{r} = 0.25 \times {10^{ - 4}}$