83. We note that every Calcium-40 atom and Krypton-40 atom found now in the sample was once one of the original number of Potassium atoms. Thus, using Eq. 43-13 and Eq. 43-17, we find
NK ln = -t NK + NAr + NCa
1 ln 2 ln = - t 1+1+8.54 T1/2 which (with T1/2 =1.26 × 109 y) yields t =4.3 × 109 y.