Statistic and Probability

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In an exam taken by a large number of students, the scores (assumed to be continuous) have a normal distribution with mean 70 and variance 100. The examiners decide on the following scale: . [90, 100] - A . [80, 90) - B . [60, 80) - C . [50, 60) - D . [0, 50) - Fail Given that a randomly chosen student received a grade of C or higher, find the conditional probability that that student received a grade of B