39. The number Nof undecayed nuclei present at any time and the rate of decay Rat that time are related by R = λN, where λis the disintegration constant. The disintegration constant is related to the half-life T1/2by λ= (ln 2)/T1/2, so R= (Nln 2)/T1/2and T1/2= (Nln 2)/R. Since 15.0% by mass of the sample is
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