If 6/25 students usually receive an A on the history test you can do 6/25=x/125.
You notice that 25 can go into 125, 5 times meaning that it has to be 6*5 which is 30.
If they are asking for frequency, this would be the answer: About 30 students
If they are asking for relative frequency, this would be the answer: About 30/125