To find the mean of the numbers 10, 5, 8, and 13, you follow these steps:

1. Add the numbers together: $10 + 5 + 8 + 13 = 36$

2. Count how many numbers there are: There are 4 numbers.

3. Divide the sum by the number of values: $\frac{36}{4} = 9$

So, the mean is 9, which corresponds to option C.

Would you like a more detailed explanation or have any other questions?

Here are 5 related questions to expand your knowledge:

1. How is the mean affected if one of the numbers is changed?
2. What is the difference between mean, median, and mode?
3. How do you find the mean of a larger set of numbers?
4. How do you calculate the weighted mean if some values are more important than others?
5. Can the mean of a set of numbers ever be larger than the largest number in the set?

Tip: When calculating the mean, always ensure you've included all numbers and divided by the correct count.