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1. The half-life of a radioactive substance is 30 years. For an initial amount of 5000 grams of the substance, complete the following table for some values of the amount of the substance left after t years. 1 years 0 30 60 90 120 150 amount left 5000 gms (a) Determine the exponential equation that gives the amount y left after 1 years. (b) How much of the original 5000 grams of the substance is left after 100 years? (c) How long has it been since the initial time if the amount left is 75 grams?