Conditional Probability
Given two events A and B (not excluding), the probability that A occurs given that B has occurred is called a <em>conditional probability</em> and is calculated as:
Where P(A∩B) is the probability that A and B occur simultaneously and P(B) is the probability that B occurs.
Now with the given data, we must find the values of the required probabilities.
30% of the students play a sport (S), this means that:
70% of the students don't play a sport (NS).
65% of the students have a job.
Note that there could be students who both play sports and have a job.
Of the 30% of the students who play a sport, 50% have a job. This means that:
15% of the students play a sport and don't have a job
15% of the students play a sport AND have a job
65% - 15% = 50% of the students have a job and don't play a sport
That last number is the numerator of the equation given above:
P(A∩B) = 0.5
The event B corresponds to students that don't play a sport (NS), thus:
P(B) = 0.7
Thus we have:
The required probability is 5/7 or 0.7143