Answer
28.6 years
Work Step by Step
Recall that $\ln(\frac{R}{R_{0}})=-kt=-\frac{0.693}{t_{1/2}}t$
$\implies \ln(\frac{20}{100})=-\frac{0.693}{12.3\,years}\times t$
Or $t=-\frac{\ln(0.2)\times12.3\,years}{0.693}=28.6\,years$
College Physics (7th Edition)
by Wilson, Jerry D.; Buffa, Anthony J.; Lou, Bo
Published by
Pearson
ISBN 10:
0-32160-183-1
ISBN 13:
978-0-32160-183-4
Chapter 29 - The Nucleus - Learning Path Questions and Exercises - Exercises - Page 998: 23
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