Question

15.57 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 20% of the U−238 atoms in a sample of U−238 to decay? Express your answer using two significant figures. If a sample of U−238 initially contained 1.1×1018 atoms when the universe was formed 13.8 billion years ago, how many U−238 atoms will it contain today? Express your answer using two significant figures.