<PRE><B>Question 262094</B>
 A chemical sample contains 23 mg of a radioactive substance. 4 hours ago it contained 25mg. how many grams will the sample contain four hours from now? 8 hours?
formula: A(t)=Ao(1+r)^(t/f)
so I am guessing this: A(t)=25(23/25)^(t/4)and A(t)=25(23/25)^(t/8)
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Almost right.  If t=0 is the present time, then you have to start with the 23 mg that are left.
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A(4)=23(23/25)^(4/4) = 21.16 mg and 
A(8)=23(23/25)^(8/4) = 19.4672 mg
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