Measurements indicate that 27.83% of all rubidium atoms currently on the earth are the radioactive 87Rb isotope. The rest are the stable 85Rb isotope. The half-life of .7Rb is 4.75 x 1010y. Assuming that no rubidium atoms have been formed since, what percentage of rubidium atoms were 87Rb when our solar system was formed 4.6 x 109y ago?