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In an entrance examinations there are mu...

In an entrance examinations there are multiple choice question. There are five possible answers to each question of which one is correct. The probability that a student knows the answer to a question is 80%. IF he gets the correct answer to the question, then the probability that he was guessing is

A

`1/9`

B

`1/10`

C

`1/21`

D

`1/20`

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The correct Answer is:
C
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