#### Answer

Dan's speed when he lands on the ground is 3.6 m/s

#### Work Step by Step

We can use conservation of momentum to find Dan's final speed $v_f$. Let's consider the system of Dan and the skateboard. Let $m_D$ be Dan's mass. Let $m_s$ be the mass of the skateboard.
$p_f = p_0$
$m_D~v_f+m_s~(8.0~m/s) = (m_D+m_s)~v_0$
$v_f = \frac{(m_D+m_s)~v_0 - m_s~(8.0~m/s)}{m_D}$
$v_f = \frac{(50~kg+5.0~kg)(4.0~m/s) -(5.0~kg)(8.0~m/s)}{50~kg}$
$v_f = 3.6~m/s$
Dan's speed when he lands on the ground is 3.6 m/s.