$T _{\max }=$ Breaking stress $\times$ Area
$=5 \times 10^{8} \times 10^{-4}=5 \times 10^{4}$
$20\,V ^{2}=5 \times 10^{4}$
$V=\sqrt{\frac{1}{4} 10^{4}}=50\,m / s$

[Area of cross section of wire $=0.005 \mathrm{~cm}^2$, $\mathrm{Y}=2 \times 10^{11}\ \mathrm{Nm}^{-2}$ and $\left.\mathrm{g}=10 \mathrm{~ms}^{-2}\right]$