In a survey of 100 people, 40 were casual drinkers (event D), and 60 did not drink (event D). Of the ones who drank, 6 had minor headaches. Of the non-drinkers, 9 had minor headaches. Let D represent set of surveyed people who were casual drinkers and D' represent those who did not drink. Let H represent set of surveyed people who reported having had minor headaches, and H ' represent the set of those who did not have minor headaches.

Find P(D) (the probability that a single randomly-selected survey respondent was a casual drinker)