Probability

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Im not sure how to start here. Please help.

The age distribution of students at a community college is given below.

Age (years)
Under 21
21-25
26-30
31-35
Over 35

Number of students (f)
411
407
201
51
26

A student from the community college is selected at random. Find the conditional probability that the student is between 26 and 30 given that he or she is at least 26.


Of the 278 (201+51+26) students who are 26 or older, 201 are 26-30. That is a probability of 206/278 = 74.1%


I'm not sure I understand this. Why are we including 51, and 26 if there are only 201 students that are between 26-30. Where does 206 come from?

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