$14\mathrm{.}40$ a $50-$mm$-$by$-300$mm scaffold timber plank, placed flat$-$wise, must carry a uniformly distributed line load of $1\mathrm{.}50$ kN$/$m without exceeding an allowable bending stress of $11\mathrm{.}0$ MPa and an allowable shear stress of $0\mathrm{.}83$ MPa. Calculate the maximum allowable simple span length. Use full nominal dimensions and neglect the weight of the plank