There are 45 students in an elementary class... There are 45 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 examination paper is a random variable with an expected value of 5 min and a standard deviation of 4 min
a) if grading times are independent and instructor begins grading at 6:50 PM and grade continuously, what is the approximate probability that he is through grading before the 11:00 PM TV news begin?
for this part, I'm not sure how to approach, the thing that really throws me off is finding the probability between 6:50 through 11:00. am I supposed to find the number of minutes between these two hours? and find the probability of that?

As the number of papers gets large the distribution of the average time spent grading a paper increasingly resembles a normal distribution.  (Central Limit Theorem)  At $n=45$ it is reasonable to use the normal distribution with ($\mu = 5, \sigma = 4$).
The sum of $n$ normally distributed random variables is $n\mu$
and standard deviation is $\sqrt n \sigma$  
