Ex$.6\mathrm{.}33.$ The bax beam shown in Fig. $6\mathrm{.}55$ is made up of four $152\mathrm{.}5cm$ wooden planks connected by screws. Each screw can sajely transmit a shear force of $1250$ N $.$ Estimate the minimum necessary spacing of screws along the length of the beam if the maximum shear force transmitted by the cross$-$section is $5000$ N $.$ Sketch the corresponding shear stress distribution across the cross$-$section.

$($a$)$