Explanation:
2r - 18 < = 5r + 3
Subtract both side by 3
2r - 18 - 3 <= 5r +3 - 3
Simplify
2r - 21 <= 5r
Subtract both side by 2r
2r - 2r - 21 <= 5r - 2r
-21 <= 3r
Divide both side by 3
-21 ÷ 3 <= 3r ÷ 3
-7 <= r
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