The simply supported beam in Figure below has a rectangular cross section

$120$ mm wide and $200$ mm high. $(1)$ Determine the shear force and bending

moment diagrams; $(2)$ Calculate the maximum bending stress in the beam; $(3)$

Sketch the bending stress distribution over the cross section on which the

maximum bending stress occurs; and $(4)$ Calculate the bending stress at a

point on section $B$ that is $15$ mm below the top of the beam.

$(2$ points$)$