Naturally occurring uranium is composed mostly of 238U and 235U, with relative abundances of 99.28% and 0.72%,

Question:

Naturally occurring uranium is composed mostly of 238U and 235U, with relative abundances of 99.28% and 0.72%, respectively. The half-life for 238U is 4.5 X 109 years, and the half-life for is 7.1 X 108 years. Assuming that the earth was formed 4.5 billion years ago, calculate the relative abundances of the 238U and 235U isotopes when the earth was formed.