To determine the constant of proportionality \( \frac{y}{x} \), we first need to understand what \( y \) and \( x \) represent in this scenario.
From the problem, we know that Julian takes \( \frac{1}{2} \) hour (or 30 minutes) to walk 2 miles.
Let's define the variables:
- Let \( x \) be the time in hours.
- Let \( y \) be the distance in miles.
From the information provided:
- In \( \frac{1}{2} \) hour, Julian walks 2 miles.
So we can substitute these values into the formula for the constant of proportionality \( \frac{y}{x} \):
\[ \frac{y}{x} = \frac{2 \text{ miles}}{\frac{1}{2} \text{ hour}} = 2 \div \left( \frac{1}{2} \right) \]
Calculating the right side:
\[ 2 \div \left( \frac{1}{2} \right) = 2 \times 2 = 4 \]
Thus, the constant of proportionality \( \frac{y}{x} \) is \( 4 \).
This means that Julian walks at a rate of 4 miles per hour.