Divide the sum of all numbers (x₁ + x₂ + ... + x₃) by the amount of values n. Hence, the formula used to calculate the arithmetic mean is A = (x₁ + x₂ + ... + x₃)/n.
For example, the calculate the mean of 5 values: 12, 30, 25, 86, and 40:

1. Sum the values: 12 + 30 + 25 +86 + 40 = 193
2. Divide the sum by n values: 193 / 5 = 38.6
3. Great work! 38.6 is the arithmetic mean of 12, 30, 25, 86, and 40.

The geometric mean is calculated by taking the n^th root of the product. The formula used to find the geometric mean is: ⁿ√(x₁ × x₂ × ... × x₃).

The n^th root can also be written as 1/n, hence (x₁ × x₂ × ... × x₃)^1/n.

To calculate the mean in Excel, use the AVERAGE function. Enter your data in a column and type =AVERAGE(range) where range is the cell range of your data (e.g., A1:A10). Press Enter, and Excel will calculate the arithmetic mean for you.

On another note, you can also use the functions =GEOMEAN() and =HARMEAN() to find specifically the geometric mean or the harmonic mean.