Tell me the answer and how you got it

If p(a) = 0.70 and p(b) = 0.20, then a and b are independent events if

Delbert keeps track of total of the total number of points he earns on homework assignments, each of which is worth 60 points. at the end of the semester he has 810 points. write an equation for delbert’s average homework score a in terms of the number of assignments n.

Math i have no idea how to do a and b