a) There are $5$ different ways to obtain $\color{blue}{X}+\color{red}{Y}=8,$ namely $$\\{\color{blue}{X},\color{red}{Y}\\}=\cases{\color{blue}{2},\color{red}{6}\\\ \color{blue}{3},\color{red}{5}\\\ \color{blue}{4},\color{red}{4}\\\ \color{blue}{5},\color{red}{3}\\\ \color{blue}{6},\color{red}{2}\\\\}$$
whereof only one has a red $5$, so the probability would be $\frac{1}{5}.$
b) That either die is $5$ (still when they sum to $8$) is seen in two of the cases above, so the probability is $\frac{2}{5}.$