1. The problem statement, all variables and given/known data "K-40... decays by two radioactive processes. It can decay by electron capture or β- emission. It is found that a sample containing 4.0x10^18 nuclei of K-40 emits a total of 68 β- particles and photons each second. This shows the half life is 1.3x10^9 years." Use the data in the passage to show that the half life is 1.3x10^9 years. 2. Relevant equations I'm assuming... T1/2 = ln2/λ; A=λN; A=A0e^(-λ)(t) 3. The attempt at a solution Using A=λN --> A/N=λ --> (68)/(4*10^18) = 1.7^10-17 then ln2/(1.7^10-17) = 4.1*10^16 ->> waayy too big. I then tried various other combinations of the above, but to no success. Please help!! I know I'm missing something VERY obvious.