Assume your friend just took an exam made up of 20 true/false questions. Further assume that your friend had no knowledge relevant to the questions, and consequently just guessed the answer to each question.

What is the probability he will get an A (with 18 or more questions answered correctly)?



Answer :

lcvey
In this case, we want to find the probability of getting 18 or more correct answers out of 20, so we need to sum the probabilities of getting exactly 18, 19, and 20 correct answers.

We can calculate each of these probabilities using the formula above:
P(X=18) = C(20, 18) * (0.5^18) * ((1-0.5)^(20-18)) P(X=19) = C(20, 19) * (0.5^19) * ((1-0.5)^(20-19)) P(X=20) = C(20, 20) * (0.5^20) * ((1-0.5)^(20-20))

Answer: The final answer is the sum of these three probabilities.