Answer:

Step-by-step explanation:
We have,
Mass of a wrench, m₁ = 4 g = 0.004 kg
Speed of wrench, v₁ = -15 m/s
Mass of the Astronaut, m₂ = 70 kg
It is required to find Astronaut's velocity. Let it is v₂. Using the conservation of linear momentum to find it.

So, his velocity is
.