Here we can use momentum conservation as there is no external force on sled and child while he jump on sled
by momentum conservation equation
[tex]m_1v_{1i} + m_2v_{2i} = m_1v_{1f} + m_2v{2f}[/tex]
[tex]20*4 + 5*0 = 20*v + 5 *v[/tex]
since sled and child both moves with same speed so here they both will have same final speed "v"
by solving above equation we will have
25 v = 80
v = 3.2 m/s
So they will move together with speed 3.2 m/s