One method of dating rocks is based on their $^{87} \mathrm{Sr} /^{87} \mathrm{Rb}$ ratio. $^{87} \mathrm{Rb}$ is a $\beta^{-}$ emitter with a half-life of
$5 \times 10^{11}$ years. A certain rock has a mass ratio $^{87} \mathrm{Sr} /^{87} \mathrm{Rb}$ of $0.004 / 1.00 .$ What is the age of the rock?