I'm sure this is answered somewhere but I don't know the correct terms to use to search effectively.

Given: I am taking tests on which I can receive grades of A, B, or C. I have currently completed 45 tests 64% of which have a grade of A. I don't know how many test I will be given between now and a deadline of end of the semester.

I would like to know how to calculate the number of As I would have to receive in order to end with a percentage of 85% minimum - assuming that I don't receive any more Bs or Cs.