Q9 of 12 Page 13

Of the students in a school, it is known that 30% have 100% attendance and 70% students are irregular. Previous year results report that 70% of all students who have 100% attendance attain A grade and 10% irregular students attain A grade in their annual examination. At the end of the year, one student is chosen at random from the school and he was found to have an A grade. What is the probability that the student has 100% attendance? Is regularity required only in school? Justify your answer.[CBSE 2017]

Let A be the event given as student attained 'A' grade.

Let E1 be the event that student is regular.


and E2 be the event that student is irregular.


Required Probabilities is-

P(E1/A) = Probability that a student is regular and attained 'A' grade.
P(A/E1) = Probability that a student attained grade 'A' and is regular.
P(A/E2) = Probability that a student attained grade 'A' and is irregular.


Here,




Thus, the probability that the student has 100% attendance if he


was found to have an 'A' grade is (3/4).

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