You are asked to calibrate the activity of a Cs-137 gamma-ray source by comparison with

a standard Cs-137 reference source of approximately the same activity. The standard source

has a quoted activity of 3.50 0.05 Ci (one standard deviation) and either source alone

gives rise to a counting rate of about 1000 per second in the available counter. Background

rates are negligible. Assuming that each source is counted separately for equal counting

times, how much total time will be required to determine the unknown activity to within a

2 % expected standard deviation?