# Find the probability of this question?

A quiz consists of 100 multiple choice questions, each with five possible answers, only one of which is correct. If the student guesses on each question, what is the probability that student answered at least 30 questions correctly.

What I tried:

I used the binomial CDF formula

I tried doing the 1 - bionomial CDF of n = 100, p = 0.2 and x = 30, so

$p(x>=30) =1-p(x<30)= 1-binoCDF(100,0.2,29) = 0.0112$

but as you can see that's not right.

• The number you got looks about right (I did a quick normal approximation estimate). – André Nicolas Feb 21 '16 at 23:36
• answer says it's 0.087 though – a wee little worker Feb 21 '16 at 23:41
• We seem to agree it isn't. – André Nicolas Feb 21 '16 at 23:54