Rubidium has two principle isotopes, 85Rb (84.911789 amu) and 87Rb (86.909183 amu). What is the percentage abundance of the lower mass isotope?

(84.911789x) + (86.909183 (1 - x)) = 85.468 - - > Multiply out the brackets

84.911789x + 86.909183 - 86.909183x = 85.468 - - > Subtract the bold part from both sides

84.911789x - 86.909183x = - 1.441183 - - > Simplify the left side of the equation

- 1.997394x = - 1.441183 - - > Divide both sides by the bold part to get x

x = 0.721532 - - > Convert to percentage

= 72% (2. s. f.)