The half-life of a radioactive material is the time that it takes to reduce to half
In this case, the half-life is 21 hs, and since 42hs is twice the half-life, the material will reduce to half after 21 hours and then to half again.
one half of one half is:
[tex]\frac{1}{2}\cdot\frac{1}{2}=\frac{1}{4}[/tex]Then we multiply by the initial amount:
[tex]19,296\cdot\frac{1}{4}=4824gr[/tex]The amount left after 42 hours is 4824 grams.
