True. If the amount displaced is more than the mass, it floats. If the amount is less than the mass, it will sink.
Velocity is distance over time

. If it wants it in km/h you're good to go otherwise if its m/s you need to convert km to m by multiplying 30 by 1000 and t in hours to seconds by multiplying 0.5 by 3600.
Concave lens. These are used in making the objectives of reflection telescopes