Question

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.

Answer 1

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.