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Exercise-2 - A certain radioactive material decays at a rate proportional to the amount present. Initially there is-50-milligrams of the material present. After two hours, the amount of radioactive material has lost 10% of its original-mass." a) Calculate the value of the decay constant, A. 1 b) Find the half-life of the material." + ."-(In(45mg/50mg)/2hr)_0.0527hr + b. )=0.693/ 2->t -0.693/2=> 0.693/0.0527=13.15 his]