On July 4, 2005, the NASA spacecraft Deep Impact fired a projectile onto the surface of Comet Tempel 1. This comet is about 9.0 km across...

...Observations of surface debris released by the impact showed that dust with a speed as low as 1.0 m/s was able to escape the comet.

I'm not sure on how to solve part B! All I know is that because 70% of its intial kinetic energy is lost, there is 0.3PE

enter image description here