Whenever you have a decision tree like this, the probability of reaching a specific leaf is the multiplication of the probabilities of each step you have to reach the leaf.
So, in order to reach the "hardcover and nonfiction" leaf, we have to take the "buys hardcover" branch (probability 0.35), and then the "buys nonfiction" branch (probability 0.55).
Given what we said above, the probability for "hardcover and nonfiction" is
[tex]0.35\cdot 0.55=0.1925[/tex]
