The half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of initial concentration.
a.) How long will it take for 20% of the U-238 atoms in a sample of U-238 to decay?
b.) If a sample of U-238 initially contained 1.4×1018 atoms when the universe was formed 13.8 billion years ago, how many U-238 atoms will it contain today?