Correct answer: 143
Life of sample \(\rightarrow t\) years
\([A] _0 \propto\) Initial mole of \(\mathrm{U}-238\)
\([A]]_t \propto\) Final mole of U-238
\(\frac{[A]_0}{[A]_t} =\frac{\frac{1}{238}+\frac{7}{206}}{\frac{1}{238}}\)
\(=\frac{0.0042+0.0340}{0.0042}\)
\(=9.1\)
\(=\frac{2.303 \log 2 \times t}{4.5 \times 10^9}=2.303 \log 9.1 \)
\(t =14.27 \times 10^9 \text { years }\)
\(=142.7 \times 10^9 \text { years } \)
\(P =142.7\)
\(P =143\)