The document discusses the arithmetic mean and how to calculate it. The arithmetic mean is calculated by adding all the numbers together and dividing by how many numbers there are. As an example, the mean of 2, 7, and 9 is 6. The document also provides formulas for calculating the mean of discrete and continuous data series, where the mean of a discrete series is calculated by summing the product of each value and its frequency and dividing by the total frequency, and the mean of a continuous series uses grouped data.