(1) ANSWER: 37.04%
METHOD: The probability of selecting the correct answer by chance is ⅕=0.2 for each question. Let p=0.2 then q=1-p=0.8 (the probability of an incorrect guess).
(p+q)²⁰=1=p²⁰+20p¹⁹q+...+C(20,20-r)p²⁰⁻ʳqʳ+... for 0≤r≤20.
C(20,20-r)=C(20,r) by symmetry of the binomial expansion.
The probability of 5 or more correct is calculated by adding the first 16 terms. But it’s easier to sum the last 5 terms and subtract fro
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