# The element rubidium (symbol Rb) contains two isotopes, "^85Ru and "^87Ru, with percentage abundance of 72% and 28% respectively. What is the relative atomic mass (A_r) of B?

${A}_{\text{rubidium"=0.72xx85+0.28xx87=85.6*"amu}}$.
Note that $R u$ is the symbol of $\text{ruthenium}$.