Step-by-step explanation:
what is the criteria for success ? how many questions must be right ? and how many must be wrong for failure ?
if success means all answers right, then she has 4 choices on the first question to pick one right answer. and then for each of those again 4 choices on the second question and so on.
so, all possible outcomes are 4²⁰.
that means the probability to guess all 20 right is
1/4²⁰
a tiny, tiny number.
and the probabilty to have all wrong ?
she has 4 choices to pick 1 of 3 wrong answers.
so, the probability is 3/4 to answer the first question wrong.
for that she has again then the same chance to get the second question wrong too.
so, it is 3/4 × 3/4 = 9/16
and so on.
the probability to guess all 20 wrong is then
(3/4)²⁰ ≈ 0.0032
that is still a small number but much, much larger than the probability to get everything right.
still, even the goal to truly get everything wrong is highly unlikely.
