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liubo4ka [24]
3 years ago
6

A multiple-choice test has 30 questions and each one has five possible answers, of which only one is correct. If all answers wer

e guesses, find the probability of getting exactly four correct answers.
Mathematics
1 answer:
kumpel [21]3 years ago
5 0

Answer: the probability is 0.13

Step-by-step explanation:

First, each question has 5 options and only one is correct.

Then, selecting at random, the probability of getting a correct answer is equal to:

p = 1/5 = 0.20 (and the probability of getting it incorrect is p = 4/5 = 0.80)

Now, if out of 30 questions, we got 4 correct and 26 incorrect, the probability for a given combination is;

p = (0.20^4)*(0.80^26)

But we also need to multiply this by the total number of combinations.

This is we have 30 questions in total, and we can select 4 of them that will be the correct ones.

Now, if we have N objects in total, the number of different combinations of K elements out of those N elements is

C = \frac{N!}{(N-K)!*K!}

In this case, N = 30 and K = 4.

C = \frac{30!}{(30 -4)!*4!} = \frac{30*29*28*27}{4*3*2}  = 27,405

Then the probability of getting exactly 4 correct answers is:

P = (0.20^4)*(0.80^26)*27,405 = 0.13

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