Splitting of signal into different components for calculation of PSD
Hint:

$$f_i=8+i\cdot 2\;\text{Hz},\quad i=0,1,\ldots,11$$

Note that $f_0=8\,\text{Hz}$ and $f_{11}=30\,\text{Hz}$. So you have 12 frequencies in the range $[8,30]\,\text{Hz}$, including both edge frequencies.