A hint to get started
Distinguish finite sampling from independent-person conditioning
(a) Mrs Wang bakes 15 chocolate and 25 blueberry cupcakes, then gives away 8 chosen at random. Find the probability exactly 3 are chocolate. marks
(b) In a seasonal-flu exercise, the probability a person receives a flu vaccine is 0.85. The probability of flu is 0.23 given vaccination and 0.91 given no vaccination. Find the probability that
(i) a person gets flu, marks
(ii) of two randomly chosen people who got flu, exactly one received the vaccine. marks
[2014/NJC/II/9]
CJC · Review problems · Question 14
For the cupcakes, counts change without replacement. For (b), use the supplied conditional population rates.