How many years are required for 28 mg of cesium-137 to decay to 3.5 mg of cesium-137?
The half life of cesium-137 is approximately
#log_2 (28/3.5) = log_2 (8) = log_2 (2^3) = 3#
So it will take
#3 * 30.17 ~~ 90.5#
This video shows a sample of Cs-137 being tested by a hand-held detector.