If y varies directly with x, and y = 18 when x = 4, what is the constant of proportionality?

Several paint mixtures are made by mixing blue paint and white paint. paint colors blue paint white paint mixture a 5 cups 12 cups mixture b 6 cups 6 cups mixture c 4 cups 12 cups mixture d 5 cups 6 cups which mixture has the highest ratio of blue paint to white paint