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We list some radioactive isotopes and their associated half-lives. Assume that each decays according to the formula A(t) = A0e kt where A0 is the initial amount of the material and k is the decay constant. For each isotope:  Find the decay constant k. Round your answer to four decimal places.  Find a function which gives the amount of isotope A which remains after time t. (Keep the units of A and t the same as the given data.)  Determine how long it takes for 90% of the material to decay. Round your answer to two decimal places. (HINT: If 90% of the material decays, how much is left?)

PROBLEM: Uranium 235, used for nuclear power, initial amount 1 kg grams, half-life 704 million years.