On your first two exams, your grades are 86% and 88% x=86 and y=88 What must you get on the third exam to have an average of 90%?
(86 + 88 + z)/3 = 90
86 + 88 + z = 270 etc
To calculate the grade you need on the third exam to achieve an average of 90%, you can use the formula:
( (x + y + z) / 3 ) = 90
Where:
x = grade on the first exam (86%)
y = grade on the second exam (88%)
z = grade required on the third exam
Now plug in the values:
( (86 + 88 + z) / 3 ) = 90
Multiply both sides of the equation by 3 to get rid of the fraction:
86 + 88 + z = 270
Combine like terms:
z = 270 - 86 - 88
z = 96
Therefore, you must get a grade of 96% on the third exam to have an average of 90%.