Isotope-X is an unstable isotope that decays to isotope-Y with a half-life of 1.5 billion years, and that is the only way to produce isotope-Y. You...

Isotope-X is an unstable isotope that decays to isotope-Y with a half-life of 1.5 billion years, and that is the only way to produce isotope-Y.

You observe a certain star and find that by number, 0.01% of its atmosphere is made of isotope-X and 0.31% is made of isotope-Y.

How old is the star?