A particle $P$ is projected from a point on the surface of a smooth inclined plane (see figure). Simultaneously another particle $Q$ is released on the smooth inclined plane from the same position. $P$ and $Q$ collide on the inclined plane after $t=4 \mathrm{s}$. Find the speed of projection in $\mathrm{m} {\mathrm{s}}^{-1}$. (take $g=10 \mathrm{m} {\mathrm{s}}^{-2}$)