The half-life of rubidium-87 (natural absorbance 27.83%) is 5.1x10^11 years. What mass of rubidium is present if an average sample decay rate of 35.0 disint./sec is detected?

Question

I've already converted years to seconds but not sure what to do after..

Answer 1

Not sure what the absorbance has to do with things, but
a half-life of 5.1*10^11 years is a decay rate of
1.359*10^-10 % per year = 4.31*10^-18 % per second
so, if 35 per second is 4.31*10^-20 of the number of atoms, that means that you have
8.05*10^20 atoms, or 0.00133 moles

Not sure whether this helps ...