Two stones, $S_1$ and $S_2$, are thrown simultaneously from points $C$ and $D$ which are 80 m apart on horizontal ground. Assume $C$ is to the left of $D$. Stone $S_1$ is thrown from $C$ towards $D$ with speed 20 ms^-1 at an angle of $60^{\circ }$ above the horizontal. Stone $S_2$ is thrown from $D$ towards $C$ with speed $U{ms}^{-1}$ at an angle of $\theta$ above the horizontal. The stones collide 2 seconds after being thrown. Gravity is the only force acting on the stones once they are in the air.