1. The problem statement, all variables and given/known data Measurements of a certain isotope tell you that the decay rate decreases from 8337 decays/minute to 3174 decays/minute over a period of 5.00 days. 2. Relevant equations R=Ro.e^-lamda/t then T1/2 = ln(2)/lamda 3. The attempt at a solution I converted all the units.. t = 5 days = 432000 seconds R = 8337 decays/minute = 52.9 decays/s Ro=3174 decays/minute = 138.95 decays/s I did: R=Ro.e^-lamda/t 52.9=138.95 . e^-lamda*432000 lamda = -1.86*10^-6 I am confused.. the lamda should not be a negative number right? because half-life should be a positive number.. please help me..