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A radioactive isotope showed a count rate of 82 counts per second initially. After a time of 210 seconds, the count rate dropped to 19 counts per second. The average background countremained constant at 10 counts per second. What is the half-life of the material?

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Actual rate = 82-10 = 72c/s; 19-10 = 9c/s.
72 →36→18→9 
210s = 3 half lives
t ½ = 210/3 = 70sec

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