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