A $1\mathrm{.}5"$ high by $2"$ wide rectangular bar is attached to a channel section by $\frac{3}{8}"$ diameter machine screws on $6"$ centers, as shown in the diagram below. The moment of inertia of the entire cross section about the horizontal neutral axis is $I=11\mathrm{.}20$ in$.{?}^4$ If the section transmits a vertical shear of $4$ kips, determine:

a$)$ the shearing stress in the machine screws,

b$)$ the shearing stress at point $A,$ and

c$)$ the maximum shearing stress in the cross section.