Chapter 41, Problem 85GP

Almost all of naturally occurring uranium is ${}_{92}^{238}\text{U}$ with a half-life of 4.468 × 10⁹ yr. Most of the rest of natural uranium is ${}_{92}^{235}\text{U}$ with a half-life of 7.04 × 10⁸ yr. Today a sample contains 0.720% ${}_{92}^{235}\text{U}$. (a) What was this percentage 1.0 billion years ago? (b) What percentage of ${}_{92}^{235}\text{U}$ will remain 100 million years from now?