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 first examination paper is a random variable with an expected value of 6 min and a standard deviation of 6 min.

If a grading times are independent and the instructor begins grading at 6:50 P. M. and grades continuously, the (approximate probability that he is through grading before the 11:00 P. M. TV news begins is probability is .