The mean is found by adding all the values together and dividing by how many there are:
$$\text{mean} = \frac{\text{sum of values}}{\text{number of values}}$$
Example: Find the mean of 3, 7, 4, 8, 3.
$$\text{mean} = \frac{3+7+4+8+3}{5} = \frac{25}{5} = 5$$
Reverse mean questions
Sometimes you're given the mean and asked to find a missing value. The key is to work out the total first:
$$\text{total} = \text{mean} \times \text{number of values}$$
Example: The mean of five numbers is 6. Four of the numbers are 3, 8, 5, and 7. Find the fifth number.
Total must be $6 \times 5 = 30$.
The four known numbers sum to $3 + 8 + 5 + 7 = 23$.
So the fifth number is $30 - 23 = 7$.