To find the constant of proportionality, we can use the relationship between the time spent (in days) and the number of swings built. The constant of proportionality \( k \) can be calculated using the formula:
\[ k = \frac{y}{x} \]
Using the data points provided:
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For \( x = 5 \) days and \( y = 1 \) swing: \[ k = \frac{1}{5} = 0.2 \]
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For \( x = 10 \) days and \( y = 2 \) swings: \[ k = \frac{2}{10} = 0.2 \]
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For \( x = 15 \) days and \( y = 3 \) swings: \[ k = \frac{3}{15} = 0.2 \]
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For \( x = 20 \) days and \( y = 4 \) swings: \[ k = \frac{4}{20} = 0.2 \]
In all cases, the constant of proportionality is \( 0.2 \).
Therefore, the constant of proportionality is 0.2 swings per day.