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Question

A student is given a true-false exam with 10 question. If he gets 8 or more correct answers, he passes the exam. Given that he guesses at the answer to each question, the probability that he passes the exam, is

A
6128
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B
9128
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C
7128
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D
none of these
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Solution

The correct option is C 7128
n= total number of ways =210=1024
Since each answer can be true or false.
and m= favorable number of ways =10C8+10C9+10C10=45+10+1=56
Since to pass the exam, he must give 8 or 9 or 10 true answers,
Hence, p=mn=561024=7128

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