Question 999055
The mean in this case divides the total points by the total number of students.
Here the total points are 12(60) + 20(50) + y(x) = 1720 + xy
The total number of students  is 12 + 20 + y = 32 + y
So that the mean is the quotient
{{{(1720 + xy)/(32 + y)}}}