Answer
$2v$
Work Step by Step
We are given that $m_1=4m_2$ and
$K.E_2=K.E_1$
$\implies \frac{1}{2}m_1v^2=\frac{1}{2}m_2(v^{\prime})^2$
$\implies \frac{1}{2}4m_2v^2=\frac{1}{2}m_2(v^{\prime})^2$
This simplifies to:
$v^{\prime}=2v$
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