A sample of a radioactive isotope contains 
nuclei at time 
. At time 
, it contains 
nuclei of the isotope. For the period 
, state, in terms of , 
and ,
the mean activity of the sample,
the probability of decay of a nucleus.
A cobalt-60 source having a half-life of 5.27 years is calibrated and found to have an activity of 
. The uncertainty in the calibration is 
. Calculate the length of time, in days, after the calibration has been made, for the stated activity of to have a maximum possible error of 
.
.
.
