Question 892639
Another recently solved problem on exponential decay will help you to learn to understand the one you asked.  Both your current question and that recently solved example deal with half-life.

Here is a link to that problem and its solution:
<a href="http://www.algebra.com/algebra/homework/Exponential-and-logarithmic-functions/Exponential-and-logarithmic-functions.faq.question.892641.html">http://www.algebra.com/algebra/homework/Exponential-and-logarithmic-functions/Exponential-and-logarithmic-functions.faq.question.892641.html</a>


When you understand the solution process, you will know what to do with question 892639, your current question.