b
(b)If target is at rest then final velocity of bodies are
\({v_1} = \left( {\frac{{{m_1} - {m_2}}}{{{m_1} + {m_2}}}} \right)\;{u_1}\) …(i) and \({v_2} = \frac{{2{m_1}{u_1}}}{{{m_1} + {m_2}}}\)…(ii)
From (i) and (ii) \(\frac{{{v_1}}}{{{v_2}}} = \frac{{{m_1} - {m_2}}}{{2{m_1}}} = \frac{2}{5}\)\(⇒\)\(\frac{{{m_1}}}{{{m_2}}} = 5\)
