How do I calculate the atomic mass for this isotope?

[apchemstudent]

How would you go about to solve this quesiton? I thought that to calculate this i had to set up an equation as:

let x = the percentage of 85-Rb

84.91*x + 86.91*(1-x) = 85
x = 96%, but that's obviously wrong.


How do i solve this question?

 

[GCT]

This may work

$$\frac{84.91x+86.91y}{x+y}=85$$

solve for $$x/y$$, or the x in terms of y

find $$(100)x/(x+y)$$

 

[apchemstudent]

This may work

$$\frac{84.91x+86.91y}{x+y}=85$$

solve for $$x/y$$, or the x in terms of y

find $$(100)x/(x+y)$$

nevermind... i know how to solve it now. It's referring to the actual weight of Rb - 85.

 

[GCT]

85.4678 is the actual weight, average weight.

http://www.webelements.com/webelements/elements/text/Rb/key.html

$$\frac{84.91x+86.91y}{x+y}=85.4678$$