Kinematics of motion in a straight line — practice question

Particle $A$ is launched vertically upward from level ground with an initial speed of $30\,\text{m s}^{-1}$. At that same instant, particle $B$ is let go from rest $15\,\text{m}$ directly above $A$. One of the particles has twice the mass of the other. As the motion proceeds, $A$ and $B$ collide and stick together to make particle $C$.