The test Linda took had 120 multiple choice questions and each question had 5 possible answers.

Question

How many questions could she have expected to have gotten right if she had guessed all of them?

What is the probability of getting at least 30 questions right by guessing all of them?

Answer 1

For each question, the odds are 1 in 5, or 20%, of guessing the right answer.

 

Assuming the questions are unrelated to each other, the odds will be the same for each question regardless of the number of questions that are answered. So, the odds will remain at 1 in 5.

 

10 May 2015 I am sorry, but I forgot the second half of your question. Here is the formula to figure the answer.

 

The binomial probabilities for situations of the general "k out of N" type are calculated through the formula:

 
P(k out of N) = (N! / (k!(N-k)!) •(p^k)(q^N-k)
where: 
N = the number of opportunities for event x to occur;
k = the number of times that event x occurs or is stipulated to occur;
p = the probability that event x will occur on any particular occasion; and q = the probability that event x will not occur on any particular occasion.

 

When you use this formula, you will be able to calculate the probability of guessing 30 answers out of 120 questions.