(લો: $\log 2=0.30 ; \log 2.5=0.40)$
$k =\frac{\ln 2}{45} min ^{-1}$
$kt =\ln \frac{1}{1-0.6}=\ln 2.5$
$\frac{\ln 2}{45} \times t=\ln 2.5$
$t=45 \times \frac{\log 2.5}{\log 2}=45 \times \frac{0.4}{0.3}=60 min$
($R$ એ વાયુ અચળાંક છે)
$log$નો ગુણધર્મ $\ln \left(\frac{{x}}{{y}}\right)=\ln {x}-\ln {y}$