Let x be the score of the 3rd quiz,, then the average score will be:
(83+91+x)/3, which = (174+x)/3. but this average should be greater or equal to 85 and smaller or equal to 90:
85 ≤ (174+x)/3 ≤ 90
1st multiply all terms with 3 → 255 ≤(174+x) ≤ 270
2nd subtract 173 from all terms → 255-174 ≤174-174+x ≤270-174
81 ≤ x ≤ 96
