Question 32908
Sounds right.  If you start with "x" amount, after
5 days you will have x/2 remaining.  After another
5 days you will have x/4 remaining.  I don't think
it is correct to say that 75% remains and is not
radioactive.  Fact is only 25% of the material remains
and it is all radioactive.
Cheers,
Stan H.