Answer :
Given data
*The given mass of the longboard is M = 4.0 kg
*Alia is rolling along on her longboard at a speed is v = 3.0 m/s
*Alia is moving at a speed is v_1 = 2.0 m/s
*Speed of the longboard relative to the ground is v_2 = 15.5 m/s
The Alia's mass is calculated by the conservation of momentum as
[tex]\begin{gathered} P_i=P_f \\ mv_1+Mv_2=(m+M)v \end{gathered}[/tex]*Here m is the Alia's mass
Substitute the known values in the above expression as
[tex]\begin{gathered} m(2.0)+(4.0)(15.5)=(m+4.0)(3.0) \\ 2m+62.0=3m+12.0 \\ m=50.0\text{ kg} \end{gathered}[/tex]Hence, Alia's mass is m = 50.0 kg