The number of gallons, y, is proportional to thw number of seconds, x, that the water is running, therefore:
[tex]\begin{gathered} y=kx \\ \text{where:} \\ k=\text{constant of proportionality} \end{gathered}[/tex]A garden hose fills a 4-gallons bucket in 20 seconds, so:
[tex]\begin{gathered} 4=k\cdot20 \\ \text{solve for k:} \\ k=\frac{4}{20}=\frac{1}{5}=0.2 \end{gathered}[/tex]Hence:
[tex]\begin{gathered} y=\frac{1}{5}x \\ or \\ y=0.2x \end{gathered}[/tex]