The average of the 3 grades = (81+85+95) /3 =87
This counts as 2/3 of the final grade:
87 * 2/3 = 58 points
For a grade of 90, they still need 90 -58 = 32 points
Let x = final exam score
The final exam is worth 1/3, so we get:
1/3x = 32
To get x, multiply both sides by 3:
x = 32 *3
x = 96
They need a 96 on the final exam.