If an ordered pair is a solution to an inequality, plug it into the inequality and simplify. If you get a true statement, then the ordered pair is a solution to the inequality. If you get a false statement, then the ordered pair is not a solution.
If you are using half as much milk every day, you'll never run out because you'll just keep using half as much. You still won't run out with the coffee shop problem or the refrigerator problem.
