# Rubidium is a soft, silvery-white metal that has two common isotopes, ""^85 Rb and ""^87 Rb. If the abundance of ""^85 Rb is 80.2% and the abundance of ""^87 Rb is 19.8%, what is the average atomic mass of rubidium,?

So, we take the weighted atomic mass of ""^87Rb and ""^85Rb.
(85xx80.2% + 87xx19.8%)"amu" = 85.40 $\text{amu}$. Check to see if this atomic mass is close to that listed on the Periodic Table, which of course considers ALL the isotopes,