Our book shows a radioactive decay series in which a sample that starts out as U-238 eventually winds up becoming non-radioactive Pb-206. The half life for this process is extremely long, being 4.51 x 10^9 years. Consider a radioactive mineral sample that is 4.64 grams of U-238 and 1.22 grams of Pb-206 found in Colorado in 1955. If all of the Pb-206 was once U-238, how old is this mineral sample? Hint: find "k" first.
1. about one billion years old
2. about 1.5 billion years old
3. about 2.3 billion years old
4. about 2.75 billion years old
5. about 7.2 billion years old