How long ago did she die if k = 1.238 * 10^-4 per year for carbon 14?

Question

This is from Chapter 9, Exercise 9-4, #40 of Fundamentals of Technical Mathematics (2nd edition): The bones of an old mathematics professor contain 99.50 percent of the original amount of carbon 14. How long ago did she die if k = 1.238 * 10^-4 per year for C 14? (Use the formula in problem 39 and find t when m/m0 - 0.9950.)

Problem #39:

http://answers.yahoo.com/question/index?qid=20071203202309AA5OEP0&r=w

Answer 1

m/m0 = e^(-kt)
where k = 1.238 * 10^(-4) per yr.

0.9950 = e^(-kt)
-kt = ln(0.9950)
t = - ln(0.9950) / 1.238 * 10^(-4)
= 40.49yr.