The mean is calculated as the sum of the observations (the sum of trees’ ages in this case) divided by the number of observations (five trees in this case). The formula is <em>X</em> = (∑<em>Obs</em>) / <em>n</em>, where <em>X</em> means the mean value, ∑ means the sum, <em>Obs</em> means the value of the observations (trees’ years), and <em>n</em> means the number of observations (5).