$(15$ points$)$ The square$-$threaded bolt and nut shown below hold together two plates. The mean radius of the thread is $7\mathrm{.}5$ mm $,$ the thread lead is $2\mathrm{.}5$ mm $,$ and the friction between the bolt and the nut is ${}_s=0\mathrm{.}35.$ The friction of the washers can be neglected.

a$.$ What moment has to be applied to the nut to create a tension of $25kN?($Result check: $M=77\mathrm{.}0N*m)$

b$.$ Once the tension has been applied, what is the magnitude of the moment to loosen the hreaded screws, fomulas