Question

A sodium Isotope has a half life of 15 hours how much sodium is left after 100 hours

Answer 1

We'll say beginning amount is 100
Ending Amount = Beginning Amount / 2^n  where n is the number of half-lives
100 hours mean six and 2/3 half-lives will elapse
Ending Amount = 100/2^(6.666666)
Ending Amount = 100 / 101.59
Ending Amount = 0.9843488532
So if we started with 100 grams after 100 hours (or 6.6666666 half-lives) we would only have 0.9843488532 grams remaining.
http://www.1728.org/halflife.htm