The half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of initial concentration. How long will it take for 10% of the U-238 atoms in a sample of U-238 to decay? If a sample of U-238 initially contained 1.5 * 10¹⁸ atoms when the universe was formed 13.8 billion years ago, how many U-238 atoms does it contain today?