Question 269482
You start with 5 possible answer choices. So if you randomly guess, 
P(correct) = .20, and P(incorrect) = .80
To omit one question means remove one answer choice that is obviously wrong. You are left with 4 answer choices, not 3 as stated in the question. So, if you randomly now guess
P(correct) = .25, and P(incorrect) = .75
Now the expression for P( n correct) is
{{{p(n correct) = (15cn)*(1/4)^(n)*(3/4)^(15-n)}}}