Answer:
mean life = 8264.5 s
Step-by-step explanation:
k = - 0.000121
The relation is given by

Now, the mean life is the life time for which the sample retains.
The mean life is the reciprocal of the decay constant.
The relation between the mean life and the decay constant is

46.2*(10-2)
10-2=8
46.2*8=369.6
Your answers listed doesnt follow along the order of pemdas. im not sure how those are the answers for that equation.
I am not going to give you the answer, but I am going to tell you how to do it. You first put 18 over 1, and multiply that by one half. (You multiply across.)
Answer:
your answer would be -135