1. Two objects $P$ and $Q$ have masses 0.6 kg and $m$ kg respectively. They are moving towards each other in opposite directions in a straight line on a smooth horizontal surface. The objects collide directly and coalesce to form an object $R$. Immediately before the collision, $P$ is travelling with speed $10m{s}^{-1}$ and $Q$ is travelling with speed $15m{s}^{-1}.$ The speed of $R$ after the collision is $12m{s}^{-1}.$

Find the value of $m$.