The activity of a radiooctine sample is measured as 4750 counts/min at $\;t=0\;$ and as 2700 counts/min at $\;t=5$ minutes . Calculate Lay life of sample

Question

$(a)\;7.2 min\qquad(b)\;6.13 min\qquad(c)\;5.24 min\qquad(d)\;8.3 min$