The question is that the human body contains 0.2% potassium by weight and the natural abundance of 40K is 0.0118%. The half life of 40K is 1.28x10^9 years and I have to calculate the activity in an adult weighing 75 kg. What I tried was to find the decay constant by 0.693/1.12x10^13 hours = 6.18x10^-14. Then I took the mass and divided it by the 40K 14.11kg/40 * 6.02x10^23 = 2.12 x10^23. Then I tried Activity=lambda*N =6.18x10^-14*2.12x10^23=1.31x10^10. The answer is supposed to be 4.39x10^-6 and I don't know what I'm doing wrong if someone could help me that would be very much appreciated.
A couple of questions: (1) If you weigh 75kg, and 0.2% of that is K, and 0.0113% of THAT is K40, do you really think there are 14kg of K40 in your body? (2) Do you really want the time constant in 1/hr? (3) If you think the final answer is 4.39E-6, what are the units of that?