Question

21% of the students had a score of 80/100 or higher on both midterms. 25% had a score of 80/100 or higher on midterm 1. what percentage of the students who had score of 80/100 or higher on midterm 1 had a score of 80/100 or higher on midterm 2

Answer 1

Let event A = high score on midterm1.
Let event B = high score on midterm2.

You want P(B|A).
  P(B|A) = P(B&A)/P(A)
  P(B|A) = 0.21/0.25
  P(B|A) = 84%

84% of students who had a high score on midterm1 had a high score on midterm 2.