Answer:
8 cups disinfectant per 36 gallons of water
Step-by-step explanation:
Yes, set up an equation of ratios:
 2/3 cup disinfectant      3 gallons water     1
-------------------------------- Â = Â -------------------------- = -----
        x              36 gallons water    12
Cross multiplication yields  12(2/3 cup disinfectant) = 1x, so that:
x = 8 cups
8 cups of disinfectant are needed to make a solution involving 36 cups water.