Thread: Conditional Probability

A class consists of seven boys and 9 girls. Two different members of the class are chosen at random. A is the event {the first person is a girl}, and B is the event {the second person is a girl}. Find the probability of:

a) B|A'

Please Help !

Originally Posted by creatively12

A class consists of seven boys and 9 girls. Two different members of the class are chosen at random. A is the event {the first person is a girl}, and B is the event {the second person is a girl}. Find the probability of:

a) B|A'

Please Help !

is asking us what if the prob a randomly getting a girl is you have already pick one.

This just reduces your sample space. After you have picked the first girl there are now 7 boys and 8 girls to choose from.

To the probability is

after thought: does mean A compliment i.e if that is the case the same reasoning as above would still work exept you would remove a boy instead of a girl from your sample space.

thnx for the reply, but why u have to take out not from a girl, but from a boy, cause A represents girls, and A' would be 1-A right, so if we say lik, 1-(9/16), then why is it wrong, can you explain this part, i'd really appreciate that

Originally Posted by creatively12

thnx for the reply, but why u have to take out not from a girl, but from a boy, cause A represents girls, and A' would be 1-A right, so if we say lik, 1-(9/16), then why is it wrong, can you explain this part, i'd really appreciate that

TES showed you an example of what to do. To paraphrase:

After you have picked the first boy there are now 6 boys and 9 girls to choose from

Therefore ....

Hello, creatively12!

A class consists of 7 boys and 9 girls.
Two members of the class are chosen at random.
= (1st person is a girl}, and = (2nd person is a girl}.

Find: .

We want: .

Since the first chosen is a boy,
. . there are 6 boys and 9 girls to choose from.
Then the probability of choosing a girl is: .

Therefore: .