Suppose that a radioactive isotope has a half-life of 5 days. If there are 200 milligrams at t = 0
and if the amount A(t) remaining after t days is given by A(t) = 200 e ^{- (.139 t)} , then

i) find amount remaining after 8 days
ii) sketch the graph of the function A for 0 < t < 20
iii) using the graph, approximate the number of days elapsed until only 75 milligrams remain.

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