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An elementary school teacher is ordering $x$ workbooks and $y$ sets of flash cards for a math class. The teacher must order at least 20 items, but the total cost of the order must not be over $80. If the workbooks cost $3 each and the flash cards cost $4 per set, which of the following systems of inequalities models this situation?<br/><br/>$$\begin{array}{c}x+y\geq 20\\ \mbox{