If the mean of 5 observations x, x + 2, x + 4, x + 6 and x + 8 is 11, then the mean of the last three observations is-

We have :
$$ \Rightarrow \frac{{x + \left( {x + 2} \right) + \left( {x + 4} \right) + \left( {x + 6} \right) + \left( {x + 8} \right)}}{4} = 11$$
$$\eqalign{ & \Rightarrow 5x + 20 = 55 \cr & \Rightarrow x = 7 \cr} $$
So, the numbers are 7, 9, 11, 13, 15
∴ Required mean
$$\eqalign{ & = \left( {\frac{{11 + 13 + 15}}{3}} \right) \cr & = \frac{{39}}{3} \cr & = 13 \cr} $$