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An examination paper has 150 multiple choice questions of one mark each, with each question having four choices. Each incorrect answer fetches -0.25 marks. Suppose 1000 students choose all their answers randomly with uniform probability. The sum total of the expected marks obtained by all these students is

  1. 0
  2. 2550
  3. 7525
  4. 9375
asked in Probability by Veteran (59.4k points) | 1.2k views

3 Answers

+32 votes
Best answer

answer = option D

Probability of choosing the correct option = $\frac{1}{4}$
Probability of choosing a wrong option = $\frac{3}{4}$

So, expected mark for a question for a student = $\frac{1}{4} \times 1 + \frac{3}{4}\times (-0.25) = 0.0625$

Expected mark for a student for 150 questions = $0.0625 \times 150 = 9.375$

So, sum total of the expected marks obtained by all 1000 students = $9.375 \times 1000 = 9375$.

answered by Veteran (342k points)
edited by
0
'Students choose all their answers randomly with uniform probability' Does this mean that it has unform distribution?
+8 votes

It is a Binomial experiment as we are doing the same bernouli experiment repeatedly. 

Which is the Bernouli experiment here ? Visiting a question and checking whether it is a correct answer or not. Why it is a Bernouli experiment ? Because a question can have either correct answer(success) or wrong answer(failure).

Total number of questions we are visiting,n = 1000*150 = 150000.

Let X be a random variable which indicates number of correct answer among the total of 150000 questions.

E(X) (i.e) Expectation of X (i.e) Expected number of correct answers = n * p  (since it is a binomial experiment)

                                    where n  is no: of Bernouli experiments and p is probability of success (i.e) probability of correct answer.

Here n = 150000 , p = 0.25

So,

     Expected number of correct answers = 150000 * .25 = 37500.

     Expected number of wrong answers = 150000 - 37500 = 112500.

Expected Total marks = Expected number of correct answers * number of marks per correct answer + Expected number of wrong answers * number of marks per wrong answer.

                                   = [37500 * 1] + [112500 * (-.25)] 

                                   = 37500 - 28125

                                   = 9375

 

 

answered by Loyal (6.8k points)
+5 votes
Expected marks per question is = -0.25 * 3/4 + 1 * 1/4 = 1/16
Since choice is uniformly distributed, expected marks = 150*1000/16 = 9375
answered by Loyal (8k points)


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