A composite beam is constructed by bolting four plates to four $60 \times$
$60 \times 12$ -mm angles as shown. The bolts are equally spaced along
the beam, and the beam supports a vertical load. As proved in
mechanics of materials, the shearing forces exerted on the bolts at $A$and $B$ are proportional to the first moments with respect to the centroidal $x$ axis of the red shaded areas shown, respectively, in parts $a$
and $b$ of the figure. Knowing that the force exerted on the bolt at $A$
is $280 \mathrm{N}$, determine the force exerted on the bolt at $B .$