You can't answer this question because you aren't giving the specific type of seismic waves. There is an s-wave a p-wave and an l-wave. Those are the basic waves. An S-wave cannot travel through a liquid at all. So, obviously it travels slower than any other seismic wave.
<span>It would travel faster because their speed depends on the density and composition of material that they pass through.</span>
You don't need to worry about the 10 year bit with this question. Just grab a calculator and divide 100/2, then the answer to that (50) by 2 etc and keep dividing by 2 until you get down to 6.25.
The answer ends up being 4 half lives :)
If you don't understand what a half life is please let me know :)