Calculate the percent activity of the radioactive isotope strontium-89 remaining after 5 half-lives.

The answer to this question would be: 3.125%

Half-life is the time needed for a radioactive molecule to decay half of its mass. In this case, the strontium-89 is already gone past 5 half lives. Then, the percentage of the mass left after 5 half-lives should be:

100% * (1/2^5) = 100%/32=3 ... 125%