Radioactive Decay?

[Tedwerd]

I am confused about the half life and radioactive decay and how to figure out the problems.

here is one from my book:

A zircon sample contains 4000 atoms of the radioactive element 235U. Given that 235U has a half life of 700 million years, how long would it take to decay to 125 atoms?

[ansgar]

smells like homework

[Rajini]

Hi,
it is simple!
Just get the formula for radioactivity..and substitute all the number that you..
tip any physics books with a topic 'radioactivity' or google 'decay law'