first of all, this has nothing to do with coursework's or homework, I'm currently revising for an exam, so please help me.

Following question:

Grade point averages of students on a large campus follow a normal distribution with a mean 2.6 and a standard deviation of 0.5.

Question a asked for the probability of a student having 3.0 or more, which is 0.2119

Question b asked for the probability of a student being somewhere between 2.25 and 2.75, which is 0.3759.

Im fine with the methods of calculating that, but now it asks the following:

c) What is the minimum grade point average needed for a students grade point average to be among the highest 10 %?

d) A random sample of 400 students is chosen from the campus. What is the probability that at least 80 of these students have a grade point average above 3.0.

For this question i think that it is an approximation for a binomial distribution (which I'm familiar with), but i don't know how to do this specifically.

Thanks!!