A student scored 82, 90, and 95 on three tests. What score must the student earn on a fourth test to have an average of 90?

To average 90 over four tests, the total of all four scores must be 4 × 90 = 360. The three known scores add to 82 + 90 + 95 = 267. The needed fourth score is 360 − 267 = 93. Average problems where you must find a missing value are solved by working from the required total: multiply the desired average by the number of items to get the total needed, then subtract the scores you already have. The student needs a 93 on the fourth test to reach a 90 average.