If there are n students and S is the score to make an 80 in the class in the final, the average score needs to be 80, so: (0.5n*88+0.1n*83.5+0.15n*80+0.25n*S)/n=80=(44n+8.35n+12n+0.25nS)/n=80. (64.35+0.25S)=80. 0.25S=80-64.35. S=15.65/0.25=62.6. So, scoring 62.6 in the final sets the grade at 80.