Divide the diameter by 2 to find the radius:

\[ r = \frac{d}{2} = \frac{0.6}{2} = 0.3 \, \text{ft} \]

Use the formula for the area of a circle:

\[ A = \pi r^2 = \pi \times 0.3^2 \approx 0.2827 \, \text{ft}^2 \]

Calculate the perimeter using the formula:

\[ P = 2 \pi r = 2 \pi \times 0.3 \approx 1.885 \, \text{ft} \]

Divide the area by the perimeter:

\[ R = \frac{A}{P} = \frac{0.2827}{1.885} \approx 0.15 \, \text{ft} \]

Choose "Plastic" from the drop-down list, which has a roughness coefficient of:

\[ C = 150 \]

Divide the height difference by the length of the pipe:

\[ S = \frac{y}{L} = \frac{6}{15} = 0.4 \]

Apply the Hazen-Williams formula:

\[ v = 1.318 \times C \times R^{0.63} \times S^{0.54} \]

\[ v = 1.318 \times 150 \times 0.15^{0.63} \times 0.4^{0.54} \approx 36.48 \, \text{ft/s} \]

Multiply the velocity by the cross-sectional area:

\[ Q = A \times v = 0.2827 \times 34.56 \approx 10.31 \, \text{ft}^3/\text{s} \]