Question 817448
( 64 + 84 + 78 + x )/4 >= 70 is the equation for obtaining at least a C
( 226 + x )/4 >= 70
Multiply each side by 4
  226 + x >= 70*4
  226 + x >= 280
Subtract 226 from each side.
        x >= 54
To determine the maximum value of x
we need to solve
( 64 + 84 + 78 + x )/4 <= 80
The rest is left for the student.