Christina's grade for her history class will be the average of her midterm exam, final exam, and final paper. She received a
91 on her midterm and an 86 on her paper. What grade must
she get on her final exam to receive a grade of 90 for the
class? Explain using R. A.C. E.

The average is calculated by the sum of the total test scores divided by the total number of tests. This can represented in the equation:

91 + 86 + x /3 = 90

177 + x /3 = 90 ⇒ Multiply both sides by 3 to get rid of the /3

177 + x /3 x 3 = 90 (3)

177 + x =

177 - 177 + x = 270 - 177 ⇒ Subtract 177 from both sides to isolate x

x =93

Therefore, they must get 93% to end with a 90% final mark