At present, concentrations of ${ }^{238}U$and ${ }^{235}U$ in natural uranium are  ${\eta }_1$ and ${\eta }_2$ and their half-lives are ${\tau }_1$  and ${\tau }_2$   to respectively. Assuming concentrations of each of these isotopes were identical at the time of the birth of the earth; find the approximate age of the earth in billion years is x. Given  ${\tau }_1=4$ Billion years and    ${\tau }_2=2$ Billion years and   ${\eta }_1/{\eta }_2=2$ .   x is ……