There are 30 students in a class that took a test. The average grade of 10 students was 80. The other 20 students average was 89. What is the overall average for all 30 students?

10 students averaged 80, so the sum of their scores was (10 x 80) = 800

20 students averaged 89, so the sum of their scores was (20 x 89) = 1,780

All together, the sum of all 30 scores was (800 + 1,780) = 2,580

30 students totaled 2,580, so their average was (2,580 / 30) = 86.