Q.

In the new pattern of IIT-JEE paper there are multiple choice questions. There are 4 possible answers to each question of which one is correct. The probability that a student knows the answer to a  question is 90% . If he gets the correct answer to a question then the probability that he was guessing is 

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a

137

b

437

c

14

d

3740

answer is A.

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Detailed Solution

A1: He knows the answer
A2 : He guess the answer
E : He gets the correct answer
PA1=910,PA2=1910=110,PEA1=1,PEA2=14
required probability=P( he was guessing /he gets the correct answer)

PA2E=PA2PEA2PA1PEA1+PA2PEA2
=110149101+11014=136+1=137

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