139. A half-life is the amount of time that it takes for a radioactive substance to decay to one half of its original quantity. Suppose radioactive decay causes 300 grams of a substance to decrease to 300 × 2^–3 grams after 3 half-lives. Evaluate 300 × 2^–3 to determine how many grams of the substance are left. Explain why the expression 300 × 2^–n can be used to find the amount of the substance that remains after n half-lives.