Answer:
[tex]v_f=0.15\:\mathrm{m/s}[/tex]
Explanation:
From the Law of Conservation of Momentum, we can write the following equation:
[tex]m_1v_1+m_2v_2=m_fv_f[/tex]
Because the toy car was initially at rest, [tex]m_2v_2=0[/tex].
Therefore, we can plug in given values and solve for [tex]v_f[/tex]:
[tex]m_1v_1=m_fv_f,\\0.050\cdot 0.20 = (0.050+0.015)v_f,\\v_f=\fbox{$0.15\:\mathrm{m/s}$}[/tex] (two significant figures).