To find the constant of proportionality for the relationship between time spent sailing (x) and distance from shore (y), we can look at the ratio of distance to time:
The constant of proportionality (k) can be calculated using the formula:
\[ k = \frac{y}{x} \]
Let's calculate it for the provided data:
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For \( x = 1 \) hour and \( y = 8 \) miles: \[ k = \frac{8}{1} = 8 \]
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For \( x = 3 \) hours and \( y = 24 \) miles: \[ k = \frac{24}{3} = 8 \]
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For \( x = 4 \) hours and \( y = 32 \) miles: \[ k = \frac{32}{4} = 8 \]
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For \( x = 9 \) hours and \( y = 72 \) miles: \[ k = \frac{72}{9} = 8 \]
In all cases, the constant of proportionality \( k \) is consistently 8 miles per hour.
Thus, the constant of proportionality is:
\[ \boxed{8} \text{ miles per hour} \]