Answer :
If it starts off at 64 grams, there are just [tex]\frac{1}{4}[/tex] grams left after 9 days.
The half-life of a radioactive element is the amount of time it takes for it to decay to half of its original value. This means that a source's activity has a half-life when it takes a certain amount of time for it to decrease to half of what it was. These radioactive atoms release energy at a quantifiable pace known as radioactive decay to transform into new, distinct sorts of atoms.
[tex]r= \frac{1}{2}\\[/tex]
[tex]a_{1} = 64\\n=9[/tex]
[tex]a_{n}= a_{1}(r)^{n-1}[/tex]
[tex]a_{9}= 64(\frac{1}{2} )^{9-1}\\a_{9}= 64(\frac{1}{2} )^{8}\\a_{9}=64(\frac{1}{256} )\\a_{9} = \frac{64}{256} \\a_{9}= \frac{1}{4}[/tex]
[tex]a_{9} = \frac{1}{4}[/tex] grams
To learn more about radioactive elements, refer:-
https://brainly.com/question/17551878
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