Probability
posted by Student .
Darby guesses the answers to six questions on the math portion of her college entrance exam. Each question is a true or false question. What is the approximate probability that at least 4 of her answers are correct?
A. 0.23
B. 0.34
C. 0.66
D. 0.78
Explanation please??

The answer is
(n,r)/2^n
Here there are 6 questions, so n=6, and
there are 2^6=64 possible sets of answers.
Since 4 of them are right, the number of combinations with 4 correct out of 6 is
(n,r)=(6,4)=6*5/(1*2)=15
What would be the probability that 4 out of six are correct if the answers are completely random?