uranium ores on earth at present time typically have a composition of $\;99.3 \%$ of isotope $\;_{92}U^{238}\;$ and $\;0.7 \%\;$ of isotope $\;_{92}U^{235}\;$. The half lives of these isotopes are $\;4.47 \times10^{9}y\;$ and $\;7.04\times10^{8}y\;$ respectively . If these isotopes were equally abundent when the earth was formed , estimate the age of earth .

Question

$(a)\;6 \times 10^{10} y\qquad(b)\;6 \times 10^{9} y\qquad(c)\;6 \times 10^{8} y \qquad(d)\;6 \times 10^{7} y$