The mean of five observations $x$, $x+2$, $x+6$, $x+4$, $x+8$ is $11$. The mean of the last three observations is:

The mean of the five observations is calculated as follows:
$$\text{Mean}=\frac{x+(x+2)+(x+6)+(x+4)+(x+8)}{5}=11$$

$$\frac{5x+20}{5}=11$$

$$x+4=11$$

$$x=7$$

The five observations are:

* $x=7$
* $x+2=9$
* $x+6=13$
* $x+4=11$
* $x+8=15$

Sorting them chronologically or by placement as listed in the text, the last three observations are $(x+6)$, $(x+4)$, and $(x+8)$, which have values $13$, $11$, and $15$.
The mean of these last three observations is:

$$\text{Mean}=\frac{13+11+15}{3}=\frac{39}{3}=13$$