Answer
She needs to have a score equal to or higher than 98 to achieve an average of 85 or higher for the four tests.
Work Step by Step
Let x represent the score of the fourth test.
Her average will be $\frac{87+81+74+x}{4}$.
Because she needs to have an average of 85 or higher for the four tests, we solve the following inequality.
$\frac{87+81+74+x}{4}$ $\geq$ 85
Multiply both sides by 4.
87+81+74+x $\geq$ 340
x + 242 $\geq$ 340
x $\geq$ 98
As we can see, she needs to have a score equal to or higher than 98 to achieve an average of 85 or higher for the four tests.
