\(\begin{array}{l}
2{v_0} = 2\left( {\frac{{{v_0}}}{4}} \right) + mv \Rightarrow 2{v_0} = \frac{{{v_0}}}{2} + mv\,2\\
\Rightarrow \frac{{3{v_0}}}{2} = mv\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( 1 \right)
\end{array}\)
Since collision is elastic \( \to \)
\( \Rightarrow v - \frac{{{v_0}}}{4} = {v_0} \Rightarrow m = \frac{6}{5} = 1.2\,kg\)