How Can We Solve GRE (Physics) Practice Problems Together?

[dinunno]

Hey, I think it would be beneficial for many if we started a thread that contained several problems (and hopefully solutions as well) that would be helpful in studying for the GRE. So, everyone, add a problem if you can (with solution!).
I'll go first:

Problem:
A sample of N atoms of helium gas is confined in a 1.0 cubic meter volume. The probability that none of the helium atoms is in a $1\times 10^{-6}$ cubic meter volume of the container is:
(a) 0
(b) (10^{-6})^N
(c) (1-10^{-6})^N
(d) 1-(10^{-6})^N
(e) 1


Solution:
(c)

 

[Reshma]

It would be more helpful if you can show how You arrived at that solution.