[b]E[/b]milia wishes to create a basic solution with 7% hydroxide (OH) ions. She has three solutions of different bases available: 10% rubidium hydroxide (Rb(OH)), 8% cesium hydroxide (Cs(OH)), and 5% francium hydroxide (Fr(OH)). (The Rb(OH) solution has both 10% Rb ions and 10% OH ions, and similar for the other solutions.) Since francium is highly radioactive, its concentration in the final solution should not exceed 2%. What is the highest possible concentration of rubidium in her solution?
1. Let \( x \) be the liters of rubidium hydroxide (Rb(OH)), \( y \) be the liters of cesium hydroxide (Cs(OH)), and \( z \) be the liters of francium hydroxide (Fr(OH)) used in the solution.

2. The total volume of the solution is \( x + y + z \).

3. The total amount of hydroxide ions in the solution is \( 10x + 8y + 5z \).

4. We need the final solution to have 7% hydroxide ions, so we set up the equation:
   \[
   \frac{10x + 8y + 5z}{x + y + z} = 7
   \]

5. To eliminate the fraction, multiply both sides by \( x + y + z \):
   \[
   10x + 8y + 5z = 7(x + y + z)
   \]

6. Distribute the 7 on the right-hand side:
   \[
   10x + 8y + 5z = 7x + 7y + 7z
   \]

7. Rearrange the equation to isolate terms involving \( x \), \( y \), and \( z \):
   \[
   10x + 8y + 5z - 7x - 7y - 7z = 0
   \]

8. Simplify the equation:
   \[
   3x + y - 2z = 0
   \]

9. Solve for \( y \) in terms of \( x \) and \( z \):
   \[
   y = 2z - 3x
   \]

10. The concentration of francium hydroxide in the final solution should not exceed 2%, so we set up the inequality:
    \[
    \frac{5z}{x + y + z} \leq 2
    \]

11. Substitute \( y = 2z - 3x \) into the inequality:
    \[
    \frac{5z}{x + (2z - 3x) + z} \leq 2
    \]

12. Simplify the denominator:
    \[
    \frac{5z}{-2x + 3z} \leq 2
    \]

13. Multiply both sides by \(-2x + 3z\) (assuming \(-2x + 3z > 0\)):
    \[
    5z \leq 2(-2x + 3z)
    \]

14. Distribute the 2 on the right-hand side:
    \[
    5z \leq -4x + 6z
    \]

15. Rearrange the inequality to isolate terms involving \( z \) and \( x \):
    \[
    5z - 6z \leq -4x
    \]

16. Simplify the inequality:
    \[
    -z \leq -4x
    \]

17. Multiply both sides by \(-1\):
    \[
    z \geq 4x
    \]

18. Substitute \( z = 4x \) into \( y = 2z - 3x \):
    \[
    y = 2(4x) - 3x = 8x - 3x = 5x
    \]

19. The total volume of the solution is:
    \[
    x + y + z = x + 5x + 4x = 10x
    \]

20. The concentration of rubidium in the solution is:
    \[
    \frac{10x}{10x} = 1
    \]

The final answer is \(\boxed{1}\) percent.