A recipe for spaghetti requires four 16-ounce bottles of ketchup to make 2 gallons of sauce. How many bottles of ketchup are needed to make 18 gallons of sauce?
Let x - number of bottles needed to make 18 gallons of sauce
bottles = 4 = x cross multiply
gallons 2 18
2x = 72 divide by 2
x = 36 bottles