Atomic mass: Isotope question

1. Apr 16, 2005

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?

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2. Apr 16, 2005

GCT

This may work

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

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

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

3. Apr 16, 2005

apchemstudent

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

4. Apr 16, 2005