The sum of 99 and 104 is:

$99 + 104 = 203$

Would you like more details or have any questions? Here are 5 more questions you might consider asking:

1. How do you solve quadratic equations?
2. What is the derivative of $x^2$?
3. Can you explain the Pythagorean theorem?
4. How do you find the area of a circle?
5. What are the properties of logarithms?

Tip: When adding two numbers, it's often helpful to break them into parts (like tens and units) to make mental calculations easier.