Question 95228
The amount of a radioactive tracer after t days is given by A(t)=Aoe^-0.18t, where A is the starting amount at the beginning of the time period. How much should be acquired now to have 40 grams remaining after 3 days?
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Let A(3) = 40 grams and solve for Ao

40=Aoe^-0.18t
40/e^-0.18(3) = Ao
Ao = 40/0.582748...
Ao = 68.64... grams is how much you would have to start with.
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Cheers,
stan H.