Ellie must mix 6 tablespoons of plant food for every 2 gallons of water. If she has 6 gallons of water, how much plant food should she use?

18 tablespoons

Explanation: for the water, you have to multiply the 2 gallons by 3 if you want to get 6 gallons. then, you also multiply 6 tablespoons by 3 to get 18 tablespoons. you can see the ratio 2:6 is the same as the ratio 6:18, so you know it’s correct!