Hello! So, what Brian did incorrectly was that he did not change the 3 into a 4, because we are looking for the mean of 4 test scores, not 3. He really needs to multiply by 4, because that would give him the right answer. The right work is 92 + 87 + 88 + x/4. Multiply that by 4 and 90 by 4 to get 92 + 87 + 88 + x = 360. 267 + x = 360. Subtract each side by 267. 360 - 267 is 93. x = 93. Brian will need a score of 93 to have a mean of 90. The strategy is to know the amount of test scores and do the steps as shown to get the right answer.
