The weight based ratio of $\;U^{238}\;$ and $\;Pb^{226}\;$ in a sample of rock is 413 . if the half life of $\;u^{238}\;$ is $\;4.5 \times10^{9}\;years \;,$ then age of rock is

Question

$(a)\;9 \times10^{9} years \qquad(b)\;6.3 \times10^{9} years \qquad(c)\;3.78\times10^{9} years \qquad(d)\;None$