The word mean in mathematical terms is basically the "central" value of a group of numbers. In other words, it is the average. To calculate the mean, you can just add up all the numbers, and divide that answer from how many numbers there are.
Example:
5, 10, 20, 30 - They all add up to 65
There are 4 numbers so you divide 65 by 4. The answer is 16.25.
