The mathematical expression for this statement can be written as:

$\frac{9}{13} + \frac{1}{2}x$

Where $x$ represents the unknown number.

Simplified expression:

$\frac{9}{13} + \frac{1}{2}x$

Let me know if you want further details on solving this expression or if you have any other questions.

Here are 5 related questions:

1. How do you solve for $x$ if the equation equals a specific value?
2. What is the result of adding two fractions with different denominators?
3. How do you solve an equation involving a fraction and a variable?
4. Can you explain the process of simplifying algebraic expressions?
5. What are some real-world applications of solving equations with fractions?

Tip: When working with equations involving fractions, it is helpful to find a common denominator to simplify the addition.