Answer
The speed of $v_{Af}$ is $~~1.9~m/s$
Work Step by Step
We can use conservation of momentum to find the velocity $v_{Af}$:
$p_f = p_i$
$m_A~v_{Af}+m_B~v_{Bf} = m_A~v_{Ai}+ m_B~v_{Bi}$
$m_A~v_{Af} = m_A~v_{Ai}+ m_B~v_{Bi}-m_B~v_{Bf}$
$v_{Af} = \frac{m_A~v_{Ai}+ m_B~v_{Bi}-m_B~v_{Bf}}{m_A}$
$v_{Af} = \frac{(1.6~kg)~(5.5~m/s)+ (2.4~kg)~(2.5~m/s)-(2.4~kg)~(4.9~m/s)}{1.6~kg}$
$v_{Af} = 1.9~m/s$
The speed of $v_{Af}$ is $~~1.9~m/s$
