Test version X of a soft drink is made up of 1 part syrup and 3 parts carbonated water. Test version Y is made up of 1 part syrup and 2 parts carbonated water. Finally, test version Z is a mixture of X and Y that has a 3:7 ratio of syrup to carbonated water. How many liters of test version X are needed to make 50 liters of test version Z?