1. A cobalt-60 source having a half-life of 5.27 years is calibrated and found to have an activity of 3.50 × 105 Bq. The uncertainty in the calibration is ±2%. Calculate the length of time, in days, after the calibration has been made, for the stated activity of 3.50 × 105 Bq to have a maximum possible error of 10%. A= -лN, where A= Activity, л=Decay Constant and N=Original number of nuclides. n=Ne^(-лt), where n=number of undecayed nuclides, N= original number of nuclides, л=decay constant and t=time. 3. The attempt at a solution: None I am utterly stumped at this one. Truthfully I didn't even understand the question properly. I can't make a connection between this uncertainty and decay. I have the solution but that didn't help me much. Here it is: source must decay by 8% A = A0 exp(–ln2 t / T½) or A/ A0 = 1 / (2t/T) 0.92 = exp(–ln2 × t / 5.27) or 0.92 = 1 / (2t/5.27) t = 0.634 years = 230 days (allow 2 marks for A/ A0 = 0.08, answer 7010 days allow 1 mark for A/ A0 = 0.12, answer 5880 days) Could someone please explain to me what exactly the question asks for, and how the quantities are related? Thanks in advance.