P$5\mathrm{.}58$ The stepped bar with a circular hole, shown in Figure P$5\mathrm{.}58,$ is made of annealed $188$ stainless steel. The bar is $12$ mm thick and a change in depth from D $=130$ mm to d $=100$ mm$.$ It will be subjected to an axial tensile load of P $=70$ kN$.$ The normal stress in the bar is not to exceed $150$ MPa. To the nearest millimeter, determine:

$($a$)$ the maximum allowable hole diameter dh$.$

$($b$)$ the minimum allowable fillet radius r$.$