Please solve it is urgent plzz plzzz

Dear Student,
Given,
Mass of the boy, m₁ = 50 kg
Speed of the boy, v₁ = 5 m/s
Mass of the trolley, m₂ = 20 kg
Speed of the trolley, v₂ = 1.5 m/s
Let their common velocity be V
Momentum of the system (boy + trolley) before the boy jumps, 
$p_1=m_1{v}_1+m_2{v}_2=50\times 5+20\times 1.5=250+30=280 kgm{s}^{-1}$
Momentum of the system after the boy jumps on the trolley
$p_2=\left(m_1+m_2\right)V=\left(50+20\right)\times V=70V$

From conservation of momentum, we get
$$\begin{gathered} p_2=p_1 \\ 70V=280 \\ V=\frac{280}{70}=4m{s}^{-1} \end{gathered}$$

Thus the common velocity is 4ms^-1.

Regards