http://www.regentsprep.org/regents/math/algebra/apr3/praccond.htm
A survey of middle school students asked: What is your favorite winter sport? The results are
summarized below:
Favorite sport winter
Grade
6th
7th
8th
TOTAL
a
.)
b
.)
c
.)
d
Snowboarding
68
84
59
211
Skiing
41
56
74
171
Ice Skating
46
70
47
163
TOTAL
155
210
180
545
What is the probability of selecting a student whose favorite sport is
skiing?
What is the probability of selecting a 6th grade student?
If the student selected is a 7th grade student, what is the probability that
the student prefers ice-skating?
If the student selected prefers snowboarding, what is the probability that
.) the student is a 6th grade student?
e If the student selected is an 8th grade student, what is the probability
.) that the student prefers skiing or ice-skating?
http://www.mth.msu.edu/~drachman/math106spring01/lec6_conditional_probability_homework_solutions.pdf
Optional (master students, optional for graduatees
In the 2000 Summer Olympics, the head of the Australian drug testing said that the odds of the
lab giving a positive result on a negative sample for an athlete was about 1 in 1000, or about : 1%.
Suppose that the tests are 100% accurate on positive samples (so they will all test positive), and
that 5% of the athletes are actually using drugs. What is the conditional probability that an athlete
who tested positive for drug use, actually had a positive sample? What would be the conditional
probability if 1% of the athletes had positive samples?
Answer: We can suppose there were 1000000 samples and work through this. Doing so, we get the
following chart for the 5% case:
s