m = amount of money you receive
You need at least $40, so that means you need m = 40 or more
So [tex]m \ge 40[/tex] which is the same as saying [tex]40 \le m[/tex]. You flip both sides and also flip the inequality sign as well.
At the same time, your parents won't give you more than $50. So the largest m can get is m = 50. This means [tex]m \le 50[/tex]
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Combine [tex]40 \le m[/tex] and [tex]m \le 50[/tex] to get the compound inequality [tex]40 \le m \le 50[/tex]
This represents all the possible values of m. We see that m can get as small as m = 40, or as large as m = 50, or anything in between.
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Answer: [tex]40 \le m \le 50[/tex]
