A quiz cosists of 20 multiplechoice questions each with 4 po

A quiz cosists of 20 multiple-choice questions, each with 4 possible answers. For someone who makes random guesses for all of the answers, find the probability of passing if the minimum passing grade is 40 %.

minimum possible passing marks - 70%.. means we need to correct >= 14 questions... therefore probability of passing = (1/4)^20 + 20C1 (1/4)^19 * (3/4)+ .... + 20C6 (1/4)^14 * (3/4)^6 = 2.951 *10^-5