# Baysian statistics and Bernoulli event

• Nov 13th 2005, 07:54 PM
lychee
help pls
1. each of three people randomly chooses one of three calculus sections to take A,B and C. what is the probability that they all choose the same one?

2. a classroom 12 boys and 10 girls in which seven students are chosen to go to the blackboard, what is the probability that the first three children chosen are boys?

thank you
• Nov 13th 2005, 08:30 PM
niva
Do you know Baysian statistics and Bernoulli event? I mean just little bit of Baysian. If you don't know what I'm talking about. You have to go read about it, before you can talk probabilities.
• Nov 13th 2005, 08:45 PM
hpe
Quote:

Originally Posted by lychee
1. each of three people randomly chooses one of three calculus sections to take A,B and C. what is the probability that they all choose the same one?

Start with something simpler: What is the probability that two people both take section A, if there are three sections? Each person has three choices, so there are three times three or nine possibilities altogether. In exactly one of these, both persons are in section A. All are equally likely, so the probability is 1/9.

Now in the problem at hand, the first person can take any section. Then what is the probability that the second and third person will enroll in the same section? Try to reduce it to the previous case. You might as well call this section "A" :)

Quote:

2. a classroom 12 boys and 10 girls in which seven students are chosen to go to the blackboard, what is the probability that the first three children chosen are boys?
It's irrelevant that seven children will be chosen. The probability that the first child is a boy is obviously 12/22 = 6/11. Now if the first child is a boy, there are 21 children left, 11 of which are boys. And so on for the third child. You can put it all together with the multiplication rule for conditional probabilities.