a) the probability that an atom of this type survives at least 5 years

b) the time at which the expected number of atoms is 10% of the original

c) if there are 1024 atoms present initially, the time at which the expected number of atoms remaining is one

d) the chance that in fact none of the 1024 original atoms remains after the time calculated in c)

Hey ya'll. The idea of half-life has always kind of confused me, so if anyone knows how to just get me started on these .... Thanks!