The points $X$ and $Y$ lie 100 m apart on horizontal ground. At time $t=0$ two balls, $M$ and $N$, are projected from $X$ and $Y$ respectively. Ball $M$ is projected with speed 30 ms^-1 at an angle of $45^{\circ }$ to the horizontal. Ball $N$ is projected with speed $U{ms}^{-1}$ at an angle of $\varphi$ to the horizontal. At time $t=2.5$, the balls collide in mid-air. Both balls are assumed to be particles in free flight.