From the conservation of linear momentum, it can be written that

$Mu=\left(M+m\right)v ...\left(1\right)$

where, $M$ is the initial mass, $u$ is the initial velocity, $m$ is the extra mass added and $v$ is the final velocity of the composition.

From equation (1), it follows that

$$\begin{gathered} 1000\times 6=1200\times v \\ \Rightarrow v=5 \mathrm{m} {\mathrm{s}}^{-1} \end{gathered}$$