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%.