A student scored 84 and 87 on her first two quizzes. write and solve a compound equality to find the possible values for a third quiz score that would give her an average between 85 and 90, inclusive

Lower score

[84 + 87 + x]/3 ≥ 85

[84 + 87 + x]/3 = ≤90

85 ≤ [171 + x]/3 ≤ 90

255 ≤ 171 + x ≤ 270

255 - 171 ≤ x ≤ 270 - 171

84 ≤ x ≤ 99

The solution is x | 84 ≤ x ≤ 99, which is the same that x belongs to [84,99]