From Mechanics

Using

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

and

\displaystyle {{v}_{A}}={{v}_{B}}=0

\displaystyle \begin{align} & 0=40\times 3+2v \\ & v=-\frac{120}{2}=-60\text{ m}{{\text{s}}^{\text{-1}}} \\ \end{align}

Skate board moves in the opposite direction to the child at \displaystyle \text{60 }\ \text{m}{{\text{s}}^{\text{-1}}}.