stats homework please help

Question

When taking a 11 question multiple choice test, where each question has 5 possible answers, it would be unusual to get X or more questions correct by guessing alone.

Give your answer for X above as a whole number.

Answer 1

To determine the probability of getting a certain number of answer right on a multiple choice test by simply guessing you would use the binomial probability formula.

Let x = number of answers correct, p = probability of getting an answer correct, n = total number of questions.

The probability of guessing the correct answer for a question when there are five choices is 1/5 or 0.2, p = 0.2.

There are 11 questions, so n = 5.

Probability of getting x questions out of 11 correct when the probability of getting each correct is 0.2:

C(11,x) * (0.2)^x * (0.8)^(n-x)