\(t = \frac{{2.303}}{K}\log \frac{a}{{a - x}}\)
\( K= \frac{{0.693}}{{t_{1/2}}}= \frac{{2.303}}{t}\log \left( {\frac{{100}}{{100 - 90}}} \right)\)
\(\therefore \,\,\frac{{0.693}}{{120}} = \frac{{2.303}}{t}\log \,10\,\)
\(\,\,\therefore \,\,t = \frac{{120 \times 2.303}}{{0.693}} = 398.8\,\) મિનિટ
(લો: $\log 2=0.30 ; \log 2.5=0.40)$