Two spheres are going in opposites directions as shown in the figure from below. The collision in progress is of inelastic nature. If the coefficient of restitution $COR=0.5$. Find the percentage of mechanical energy (with respect to the value of an instant before the collision) which is lost during the collision.

The alternatives given are as follows:

$\begin{array}{ll}

1.&50\,%\\

2.&55\,%\\

3.&60\,%\\

4.&65\,%\\

5.&75\,%\\

\end{array}$

I'm not sure exactly how should I use the information of the coefficient of restitution. Can someone help me here?.

I believe that for this situation I can use the conservation of mechanical energy as follows:

$m_1v_1+m_2(-v_2)=(m_1+m_2)v_3$

Then in order to find the kinetic energy will be found by finding $v_3$ but am I right with this?. Can someone help me?

If I replace in there the information given it would become into:

$2\times5+1\times\left(-10\right)= (2+1)v_3$

$v_3=0$

But this would give

$v_3=0$

How exactly can I find the energy lost if the speed in the end is zero?. Can someone help me with clearing out this inconsistency?.