Maria's test scores are 80, 70, and 90. What score does she need to get on a fourth test so that her average score for the four tests is exactly 85?
Think hard. Detailed solution:
100
She needs to have a total of 4 * 85 = 340 points over the four tests.

She has 80 + 70 + 90 = 240 points so far.

She needs 340 - 240 = 100 on the fourth test.

.

The final answer: 100