Two particles each of mass m, collide head on when their speeds are ${\displaystyle 4m}{{\displaystyle s}}^{-{\displaystyle 1}}$  and ${\displaystyle 2m}{{\displaystyle s}}^{-{\displaystyle 1}}$ . If they stick to each other on impact, when moving in opposite directions, their combined speed is $\left({\text{ms}}^{-\text{1}}\right)$