\(v _{1}=- v +\frac{ m _{2}}{ m _{1}} v\)
\(\frac{\left( v _{1}+ v \right)}{ v }=\frac{ m _{2}}{ m _{1}}\)
\(e =\frac{2 v }{ v _{1}}=1\)
\(v =\frac{ v _{1}}{2}\)
\(\frac{ v _{1}+ v _{1} / 2}{ v _{1} / 2}=\frac{ m _{2}}{ m _{1}}\)
\(3=\frac{ m _{2}}{ m _{1}}\)