Since Astronaut and wrench system is isolated in the space and there is no external force on it
So here momentum of the system will remain conserved
so here we can say
[tex]m_1v_{1i} + m_2v_{2i} = m_1v_{1f} + m_2v_{2f}[/tex]
initially both are at rest
so here plug in all values
[tex]0 = 100 v_{1f} + 2\times 10[/tex]
[tex]v_{1f} = -0.20 m/s[/tex]
so here the astronaut will move in opposite direction and its speed will be equal to 0.20 m/s