From Mechanics

Using

\displaystyle {{m}_{A}}{{v}_{A}}+{{m}_{B}}{{v}_{B}}={{m}_{A}}{{u}_{A}}+{{m}_{B}}{{u}_{B}}

and

\displaystyle {{u}_{A}}={{u}_{B}}

\displaystyle \begin{align} & 1000\times 20+4000\times 10=(1000+4000)v \\ & v=\frac{60000}{5000}=12\text{ m}{{\text{s}}^{\text{-1}}} \\ \end{align}