Chemistry
posted by Becky .
Geologists can estimate the age of rocks by their uranium238 content. The uranium is incorporated in the rock as it hardens and then decays with firstorder kinetics and a halflife of 4.5 billion years. A rock is found to contain 82.7% of the amount of uranium238 that it contained when it was formed. (The amount that the rock contained when it was formed can be deduced from the presence of the decay products of U−238.)
How old is the rock in years?

k = 0.693/t1/2
ln(No/N) = kt
No = 100
N = 65.7
k from above.
solve for t in years.
I found this equations, but my question is what is k? 
k is from your first line. You know halflife is 4.5E9 yrs so you can calculate k.
N however is not 65.7 but 82.7 
Yes, I just copied the equation off of a different problem.
Thank you for your help!