The mean of x, x + 2, x + 4, x + 6 and x + 8 is 11, find the mean of the first three observations.

Mean = $\dfrac{\text{Sum of all observations }}{\text{Number of all observations }}$

⇒ 11 = $\dfrac{x + (x + 2) + (x + 4) + (x + 6) + (x + 8)}{5}$

⇒ 11 = $\dfrac{x + x + 2 + x + 4 + x + 6 + x + 8}{5}$

⇒ 11 = $\dfrac{5x + 20}{5}$

⇒ 11 x 5 = 5x + 20

⇒ 55 = 5x + 20

⇒ 5x = 55 - 20

⇒ 5x = 35

⇒ x = 7

So, the observations are x, x + 2, x + 4, x + 6 and x + 8

= 7, 9, 11, 13, 15

Mean of the first three observations = $\dfrac{7 + 9 + 11}{3}$

= $\dfrac{27}{3}$

= 9

Hence, the mean of the first three observations is 9.