college
posted by poe on .
A test is composed of six multiple choice questions where each question has 4 choices. If the answer choices for each question are equally likely, find the probability of answering 3 OR 4 questions correctly.

Please type your subject in the School Subject box. Any other words, including obscure abbreviations, are likely to delay responses from a teacher who knows that subject well.

R  right
W  wrong
so we want the case of
RRRWWW or RRRRWW and its possible arrangements
P(3 R) = C(6,3)(1/4^3(3/4)^3 = .131836
P(4 R) = C(6,4)(1/4)^4(3/4)^2 = .032959
P(3R or 4R) = the sum of those two
= .1648