Question

A Biology test contains 10 multiple choice questions each with 5 choices and one correct answer. If a law school student just randomly guesses on each of the 10 questions, i.e., the probability of getting a correct answer on any given question is 0.2. Assume that all questions are answered independently. (a) What is the probability that the student answers at least 9 questions correctly

Answer 1

0.0004% probability that the student answers at least 9 questions correctly

Explanation:

For each question, there are only two possible outcomes. Either the student guesses the correct answer, or he does not. All questions are answered independently. This means that we use the binomial distribution to solve this question.

Binomial probability distribution

The binomial probability is the probability of exactly x successes on n repeated trials, and X can only have two outcomes.

`P(X = x) = C_(n,x).p^(x).(1-p)^(n-x)`

In which
`C_(n,x)` is the number of different combinations of x objects from a set of n elements, given by the following formula.

`C_(n,x) = (n!)/(x!(n-x)!)`

And p is the probability of X happening.

In this question, we have that:

`n = 10, p = 0.2`

What is the probability that the student answers at least 9 questions correctly

`P(X \geq 9) = P(X = 9) + P(X = 10)`

In which

`P(X = x) = C_(n,x).p^(x).(1-p)^(n-x)`

`P(X = 9) = C_(10,9).(0.2)^(9).(0.8)^(1) = 0.000004`

`P(X = 10) = C_(10,10).(0.2)^(10).(0.8)^(0) \approx 0`

`P(X \geq 9) = P(X = 9) + P(X = 10) = 0.000004 + 0 = 0.000004`

0.0004% probability that the student answers at least 9 questions correctly