The half-life for the radioactive decay of [U]-238 is 4.5 billion years and is independent of initial concentration. Part A How long will it take for 20% of the [U]-238 atoms in a sample of [U]-238 to decay? part b If a sample of [U]-238 initially contained 1.9×10^18 atoms when the universe was formed 13.8 billion years ago, how many [U]-238 atoms will it contain today?