A question related to radioactivity.

[DarkStalker]

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.

 

[factfinder]

CIE a level physics paper 4. the question was made to be unanswered i guess.