Final answer:
To achieve the desired 1:3 ratio of lemon juice to water in 1 gallon of lemonade, Samantha should pour out and replace 1/2 gallon of lemonade with water.
Step-by-step explanation:
To get 1 gallon of lemonade containing 1 part lemon juice for every 3 parts water, Samantha needs to know how much lemonade to pour out and replace with water. Let's assume Samantha has a gallon of lemonade. If 1 part lemon juice is equal to 1/4 gallon and 3 parts water is equal to 3/4 gallon, then she needs to replace 3/4 gallon - 1/4 gallon = 1/2 gallon of lemonade with water to achieve the desired ratio.