math
posted by jack .
In his algebra class, Rob has scores of 79, 85, 81, and 65 on his first four tests. To get a grade of C, the average of the first five tests must be greater than or equal to 70 and less than 80. Solve an inequality to find the range of scores that Rob can earn on the fifth test to get a C

Five tests with an average ≥70 means a total score of ≥ 5*70=350, but < 5*80 = 400.
He has 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. 
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