I have a problem that I've been working on FOREVER! I cannot use Algebra or anything like Trig or Calculus to solve it. I have to describe it in it's basic form. By trial and error, I think the answer is 20, but I'm also not allowed to use trial and error (guess and check). How in the world can it be solved without Algebra?

3. Two algebra classes took the same exam. The mean grade for the first class was 88. The second class had a mean grade of 79. The mean grade of the two classes combined was 84. If there were 25 students in the first class, how many were in the second class?