$5\mathrm{.}23$ A composite beam is constructed by bolting four plates to four $606012-mm$ 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 $280N,$ determine the force exerted on the bolt at $B.$