We will have the following:
[tex]\begin{gathered} (74kg)(1.54m/s)=(74kg+m_s)(1.42m/s)\Rightarrow74kg+m_s=\frac{(113.96kg\ast m/s)}{(1.42m/s)} \\ \\ \Rightarrow74kg+m_s=\frac{5698}{71}kg\Rightarrow m_s=\frac{444}{71}kg \\ \\ \Rightarrow m_s\approx6.3kg \end{gathered}[/tex]So, the mass of the skate board is 444/71 kg, that is approximatly 6.3 kg.
