The ductile steel pipe $\backslash (\backslash$left$(\backslash$sigma$\_\{$y$\}=250\backslash$mathrm$\{$MPa$\}\backslash$right$)\backslash )$ has an outer and inter diameter of $30$ mm and $20$ mm respectively, determine the factor of safety against yielding of the material at point A according to the distortion$-$energy theory.

$($A is the top farthest point on the cross$-$section as shown$)$