I'm new here, first post.

I honestly forgot the concept of half-life and this week we are having our final exams. Good thing that the teacher provided us with a review test. So would someone refresh my memory.

This is how I did A:Originally Posted byProblem 1

- P(t) = 30(1/2)^t/6
- 3 = 30(1/2)^3/6
- 3/30 = 1/2^3/6
- log (1/2)^3/30 = 3/6
- (log (3/30) / log (1/2) ) = 3/6
- (log (1/10) / log (1/2) ) = 1/2
- (log (1/10) / log (1/2) )*2 = 6.64

The problem is that my teacher said that the answer for A is 21.2 mg. I got this problem totally wrong. Can someone show me the work on how they got the answer to be 21.2 mg?

I am clueless on how to do B.

The answer to this problem is 2.5 years. I need to know how to setup the equation for this problem and how you get the answer, 2.5 years.Originally Posted byProblem 2

The answer to this problem is 20.8 mins. I need to know how to setup the equation for this problem and you get the answer, 20.8 mins.Originally Posted byProblem 3

The answer to this problem is 5700 years. How do I set this problem up and how do you get the answer, 5700 years?Originally Posted byProblem 4

Thanks for any help!