To find the constant of proportionality in the relationship between the number of pitchers of lemonade (x) and the number of lemons needed (y), we can express the relationship as \( y = kx \), where \( k \) is the constant of proportionality.
To find \( k \), we can use the given data points. We can calculate \( k \) using any of the provided pairs \((x, y)\):
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Using the pair \( (4, 28) \): \[ k = \frac{y}{x} = \frac{28}{4} = 7 \]
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Using the pair \( (7, 49) \): \[ k = \frac{49}{7} = 7 \]
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Using the pair \( (11, 77) \): \[ k = \frac{77}{11} = 7 \]
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Using the pair \( (12, 84) \): \[ k = \frac{84}{12} = 7 \]
In all cases, we find that the constant of proportionality \( k \) is 7.
Thus, the constant of proportionality is \( \boxed{7} \).