Uranium-238 and uranium-235 both decay by - particle emission. Their measured half-lives are 4.5 10 9

Uranium-238 and uranium-235 both decay by α- particle emission. Their measured half-lives are 4.5 × 10⁹ years and 7.0 × 10⁸ years respectively. At the present time the ratio of ²³⁵U to ²³⁸U in naturally occurring uranium is about 0.72%. It is believed that the uranium on Earth was formed in a supernova that took place billions of years ago. It is also believed that the ratio of ²³⁵U/ ²³⁸U produced in supernovae is roughly 1.65. Estimate how long ago the supernova that produced the uranium on Earth took place.