When the earth was rst formed from an enormous disk of gas and dust about 4.5 billion years ago (see Chapter 9), the concentration of U-235 was much higher in the earth’s crust than it is today. If...

When the earth was rst formed from an enormous disk of gas and dust about 4.5 billion years ago (see Chapter 9), the concentration of U-235 was much higher in the earth’s crust than it is today. If the concentration of U-235 in the earth’s crust today is 1.5 parts per 100 million, what was the concentration of U-235 in the earth’s crust approximately 4.25 billion years ago?