If the mean of five observations x,x+2,x+4,x+6,x+8 is 11 find the mean of first three observation.

The given observations are x,  x + 2, x + 4, x + 6 and x + 8.

∴ First three observations are 7, 9 and 11.

$\mathrm{mean} \mathrm{of} \mathrm{first} \mathrm{three} \mathrm{observations} = \frac{7 + 9 + 11}{3} = \frac{27}{3} = 9$