A projectile fired at $30^{\circ}$ to the ground is observed to be at same height at time $3 \mathrm{~s}$ and $5 \mathrm{~s}$ after projection, during its flight. The speed of projection of the projectile is ___________ $\mathrm{m} ~\mathrm{s}^{-1}$.

(Given $g=10 \mathrm{~ms}^{-2}$ )