Five tests with an average ≥70 means a total score of ≥ 570=350, but < 580 = 400.
Rob already got:79 + 85 +81 + 65 = 310 points.
What should he get to score (x) between 350 and 400?
350 ≤ 310+x < 400
Subtract 310 from the above inequality to find the range of x.