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63. The half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of the initial concentration. How long will it take for 20% of the U-238 atoms in a sample of U-2.38 to decay? If a sample of U-238 initially contained 3.21018 atoms when the universe was formed 13.8 billion years ago, how many U-2.38 atoms does it contain today