# A multiple choice examination has 5 questions. Each question has three alternative answers of which exactly one is correct. The probability that a student will get 4 or more correct answers just by guessing is

## Solution :

Probability of guessing a correct answer, p = $$1\over 3$$

and probability of guessing a wrong answer, q  = $$2\over 3$$

So, the probability of guessing 4 or more correct answers is

= $$^5C_4$$ $$({1\over 3})^4$$. $$2\over 3$$ + $$^5C_5$$ $$({1\over 3})^5$$

= $$5.2\over {3^5}$$ + $$1\over {3^5}$$ = $$11\over {3^5}$$

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