Let A be the event "randomly selected person from the 2,500 people is one who invests in municipal bonds but NOT in oil stocks" and P(A) be the probability of this event.
where
- number of desired outcomes;
- total number of outcomes that can occur.
In our case and
is equal to the number of people who invest in municipal bonds but NOT in oil stocks. That is,
shows the proportion of such people among a group of
people.
Since out of
people,
percent invests in municipal bonds, and
percent invests in both municipal bonds and oil stocks (
) percent of this group invests in municipal bonds but NOT in oil stocks. Then
of
Therefore
Redundant data (for example, the group size of 2500 and 18 percent of the group who invest in oil stocks) may confuse you, decrease your confidence in your answer and eventually slow you down. So be aware that sometimes not all data is necessary to answer a question.