The half-life of a radioactive isotope is the time it takes for a quantity of the isotope to be reduced to half its initial mass. starting with 120 grams of a radioactive isotope, how much will be left after 5 half-lives?

Each half-life reduces a sample by one half.
So after 1 half-life there is 1/2 left
2 half-lives 1/4
3 half-lives 1/8
4 half-lives 1/16
5 half-lives 1/32
There would be (1/32) of 120 remaining.
or 3.75 grams.

Source:
http://www.1728.org/halflife.htm