fgsafsf: :cry: how much of a radioactive kind of tin will be left after 20 days if the half-life is 10 days and you start with 480 grams?
You could do this the long way or the short way.
For the long way you would use the formula
N(t) = N
0e
-kt where N
0=480
N(t) = 480 * e
-kt When t=10 n=240
etc etc
OR you could be smarter and say, if the half life is 10days then
in 10days there will be 240g
in 10 days more, that is, 20days after the start, there will be 120g
+118654 
