A coworker of mine has a seventh grader who got a probability question wrong on a test, a couple of us at work are looking at it and we're getting the same answers that he got.
This is the question verbatim:
A seventh grader has 2 siblings. 1) What is the probability that this seventh grader is a girl with a brother and a sister?
2) What is the probability that there are at least 2 girls in the family?
For the first question we're getting either 1/8 or 1/6. We get 1/8 by 1/2*1/2*1/2 = p(student is girl) * p(bro) * p(sis). We get 1/6 by 1/2*1/3 = p(student is girl) * p(last 2 siblings are a boy and a girl)
For the second we're getting 1/2. We list out the combinations and half of them meet this criteria (BBB, GGG, BGG, GBB).
None of our answers are correct according to the teacher. Can someone help us out here?