To find the cost for one shirt, we can look at the total cost and the quantity of shirts for each data entry in the table.
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For 6 shirts at $18, the cost per shirt is: \[ \frac{18}{6} = 3 \]
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For 5 shirts at $15, the cost per shirt is: \[ \frac{15}{5} = 3 \]
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For 4 shirts at $12, the cost per shirt is: \[ \frac{12}{4} = 3 \]
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For 3 shirts at $9, the cost per shirt is: \[ \frac{9}{3} = 3 \]
From the calculations, the cost for one shirt in each instance is consistently \(3\) dollars.
Therefore, the cost for one shirt is $3.