Two-thirds of the people scored more than 80 points, which means that two people scored an average of 90 points.
Average score of people under 80 (1)
(85×3-90×2)÷ 1=75 (point)
Suppose there are six students in Class Two, Grade Five.
People with an average score below 80 (2 people).
(85×6-90×4)÷2=75 (points)