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-

Answer: Option B

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