Answer :

Given that, there are 20 questions of true-false exam.


If the coin falls head, he answers “true.”


If the coin falls tail, he answers “false.”


Let p be the probability of a correct answer.


That is, p = getting a head and a right answer to be “true” for a question or getting a tail and a right answer to be “false” for a question.



Then, q is the probability of the answer to be incorrect.


And, p + q = 1


q = 1 – p




Let X denote a random variable representing the number of correct answers out of 20 questions.


Then, the probability of getting r correct answers out of n answered questions is given by,


P (X = r) = nCrprqn-r


Here, n = 20 [ there are 20 questions in total]


Putting values of n, p, and q in the above equation, we get




…(i)


We need to find the probability that he answers at least 12 questions correctly.


This can be represented as,


Probability = P (X ≥ 12)


It can be written as,


P (X ≥ 12) = P (X = 12) + P (X = 13) + P (X = 14) + P (X = 15) + P (X = 16) + P (X = 17) + P (X = 18) + P (X = 19) + P (X = 20) …(ii)


Put r = 12, 13, 14, 15, 16, 17, 18, 19, 20 in equation (i) subsequently and substitute in (ii), we get





, we have got the required probability.


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