Question

silvia's test scores are 86,81 and 92 what must she score on her fourth test in order to have an average of 90 for all four tests

Answer 1

90 * 4 = 360 (total of 4 tests for average of 90)

86 + 81 + 82 = 259 (total first 3 tests)

360 - 259 = 101

answer:
In order to have an average of 90, she must score 101 on her 4th test.

Answer 2

Silvia has to score a 101 to get an average of 90 for all four tests.
Explanation: 90=(86+81+92+x)/4
                     360=259+x
                     101=x