SOLUTION: How many liters of a 15% saline solution must be added to 8 liters of a 4% saline solution to produce a 7% solution? note: Do NOT solve the entire problem. Answer only part (a) and

1.  From Wikipedia, the free encyclopedia 
    https://en.wikipedia.org/wiki/Saline_(medicine)


    Saline, also known as saline solution, is a mixture of sodium chloride in water and has a number of uses in medicine.
    Applied to the affected area it is used to clean wounds, help remove contact lenses, and help with dry eyes. 
    By injection into a vein it is used to treat dehydration such as from gastroenteritis and diabetic ketoacidosis. 
    It is also used to dilute other medications to be given by injection.


    Normal saline (NSS, NS or N/S) is the commonly used phrase for a solution of 0.90% w/v of NaCl, 308 mOsm/L or 9.0 g per litre.



2.  Such thing as "pure saline" does not exists in the nature 

         (in my opinion. I am not a chemist (studied chemistry at my high school), but this combination 
          of words "pure saline" is getting hard on the ears.)


    To check myself, I asked GOOGLE with this combination of words "pure saline".

    GOOGLE gave me "~ 8000 references" in responce.

    I know that if I ask GOOGLE about some existing thing, it will give me ~ 1 000 000 (millions) references in response.

    8000 reference is a clear evidence/indication that such thing does not exist, and all these 8000 references are "the noise" 
    due to imperfectness of the system.



3.  I suspect that in reality you wanted to ask about "pure salt amount" dissolved,

     but then it must be written accordingly.