Definition
Arithmetic mean denoted by μ, is the central value in a finite number of values. It is also known as average. Average is calculated by dividing Sum of all values by the total number of values.

Formula
𝜇=(∑X)/𝑁

MS Excel
In MS Excel, mean is calculated using "=Average(Range)" function. For example in the below demonstration a dataset of 2,3,5,8,9,11 has been defined in range A2:A7. In cell A8, we entered function "=Average(A2:A7)" and the result is 6.33 which is the arithmatic mean of all those values.
For better understanding you can draw a line chart of both data and its mean. Create another column next to data by repeating mean value (6.33) in front of each data value. Now select both columns including their headings and click on line chart in Insert>Charts tab. Now you can see both data set and average value drawn on a line chart.

Python
In python you can use "statistics" liberary to perform arithmetic mean.
If you do not have python already installed, please download one with spider. Simplest ones are portable ones which do not need to be installed like winpython.
1. To import statistics type "import statistics as st"
2. You will also need pyplot liberary to draw a chart. Import pyplot using "import matplotlib.pyplot as pp"
3. Now define your data set as variable x using "x = [2,3,5,8,9,11]"
4. Now calculate Mean and store its value in variable "m", using "m = st.mean(x)"
5. You can display the value of mean computed above using print command. Type "print("Mean: " + str(m))"
6. You can plot dataset on a line chart using "pp.plot(x,x)"
7. You can highlight the value of mean using "pp.plot(m,m,'o')"
8. In Spider, press Run and see the results. 