Answer
The speed of each particle after the collision is $4.00~m/s$
Work Step by Step
According to Equations (9-75) and (9-76), when $m_1=m_2$ in an elastic collision, then $v_{1f} = v_{2i}$ and $v_{2f} = v_{1i}$
That is, the velocities are exchanged after the collision.
Therefore, the speed of each particle after the collision is $4.00~m/s$.
