There are 40 students in an elementary statistics class. On the basis of years of experience, the instructor knows that the time needed to grade a randomly chosen paper from the first exam is a random variable with an expected value of 6 min and a standard deviation of 6 min. (a) If grading times are independent and the instructor begins grading at 6:50 p. m. and grades continuously, what is the (approximate) probability that he is through grading before the 11:00 p. m. TV news begins