.
Let's assign a few parameters:
#z=# total number of students.
#x=# the number of students that got a score of #80/100# or higher on midterm #1#.
#y=# the number of students that got a score of #80/100# or higher on midterm #2#.
Now, we can say:
#x/z=0.25, :. x=0.25z#
#y/z=0.21, :. y=0.21z#
The percentage of the students who had a score of #80/100# or higher on midterm #1# also got a score of #80/100# or higher on midterm #2# is:
#y/x=(0.21z)/(0.25z)=21/25=84/100=84%#
#84%# of the students that scored #80/100# on midterm #1# also scored #80/100# or higher on midterm #2#.