posted by Lori .
A student takes a 20-question, true/false exam and guessed on each question. Find the probability of passing if the lowest passing grade is 15 correct out of 20.
What you want to know is the chance probability of obtaining 15, 16, 17, 18, 19, or 20 correct.
Prob. of 15 = .5^15...
Prob. of 20 = .5^20
Either-or probability = sum of individual probabilities.