Hello! 1. The problem statement, all variables and given/known data Here is the problem I can't solve, and will be grateful for your help on this - please, guid me to the understanding of how to solve such problems. The difficulty lies in replacing some of the solution with water. It would be much easier to solve if we add water. How much of a 5 gallon 40% salt solution should be replaced with pure water to obtain 5 gallons of a 15% solution? I have to solve it by creating a system of linear equations. 2. Relevant equations 3. The attempt at a solution Here are my initial thoughts, but I couldn't go beyond: If we have 5 gallons of 40% salt solution, hence we have 2 gallons of salt in 5 gallons. The final solution should contain 0.75 gallons of salt (5 x 0.15); therefore, I have to replace X amount of 5 gallons of 40% solution with Y amount of water to get 5 gallons of 15% solution. X + Y = 5 Thank you!